Needless to say, this would make quite a lot of bookeeping for the GM. There are some interesting ways that this concept can be used in a campaign, though.
Unlike normal angels, they cannot petition the Seraphim Council for a different Word -- the concepts they embody are so totally ingrained in their being that nothing short of death can part them. Temporary angels do not belong to any Choir, and do not have Divinity as normal angels would understand it; for purposes of moving through the three worlds, treat their Divinity as 6. Though they may ascend to the heavens, most of their time will be spent on Earth, trying to extend their brief lives.
Temporary angels cannot Fall; they seldom live long enough to have moral questions or qualms about their tasks. This makes dealing with them even more dangerous, because in lending them aid an angel may be risking what the other negotiator doesn't have, his Divinity. This also makes it just as likely that a demon would work for one of these beings, for whatever reason.
The temporary celestials that PCs would be interested in dealing with are extraordinarily powerful, capable of regenerating many points of Essence every day. Unfortunately, for all their power they are still single individuals with impossible jobs.
That's where other characters come in. For a permenant expenditure of a small amount of Essence, usually three to five points, characters can become temporary servants of the temporary angels. These transient beings will provide special methods of Essence regeneration in exchange for service and loyalty.
To be in sync with the disposable angels -- to be able to make initial contact with them through the Symphony, if they've never met before -- a character has to have been in the corporeal realm for one week, minus a day for each corporeal Force he has. This reflects the time spent picking out the ephemeral strain he's looking for from the Symphony at large.
Remind the characters that they should be in dire straits before they start negotiating with these beings, because of what will be asked in return. Most Archangels (like Gabriel, Michael and Khalid, one of the few things they agree upon) frown upon this temporary obediance demanded by the transitory angels, but some (like David, Eli and Novalis) acknowledge their own resources to be limited, and can grudgingly ignore a bit of moonlighting if it helps their servants get their greater jobs done.
After contact has been made, the character must convince the angel he is worthy of serving him, either by recounting things he has done in the past that could have furthered the being's Word, or by completing a brief quest.
Once swearing loyalty to the temporary Word, characters will no longer regenerate Essence naturally, one per day, but will have access to all the angel's privileged methods of Essence regeneration. They will still be able to use their Superior's priviliged methods of Essence regeneration.
When a temporary angel dies, all his servants are loosed of their responsibility to him. They will regenerate Essence naturally again, and can no longer use his privileged methods of Essence regeneration.
The second week of August, the scattered servants of the angel of Autumn have celebrations on every continent to herald his (or, not infrequently, her) rebirth. Everyone listens to the Symphony for the first reverberations signalling his return, and when he makes his first corporeal manifestation, groups of them seek him out to make offerings, educate him on the current state of heavenly affairs, or ask for assistance.
Then comes the fun, as the servants of Summer meet the servants of Autumn. The longer it takes for the angel of Autumn to come into his own and raise enough power to do battle with and destroy the (by then weary) angel of Summer, the longer before the seasons truly change.
The angel of Autumn is always composed of 16 Forces, and will have an intuitive grasp of all the major Songs at level 2 or greater. He is the angel of the harvest, and since he's powerful enough to usher the season through by himself, his servants are mainly there to insure a smooth transition from Summer, and to help fight the change to Winter....
Every 10 acres of field harvested +1 Every servant of Summer slain +3 Every servant of Winter slain +4
Disposable angels serve no Archangel; they have their own priviliged methods of Essence regeneration. If a player wants to be a transitory celestial, he should first discuss with the GM what Word he wants to be an incarnation of, and the two should decide whether the role is static (a unique situation that will never occur again) or cyclic (a recurring theme that could be continued throughout a campaign). Then agree on a rough deadline.
The player should build the character on 15 Forces, and hammer out three methods of Essence regeneration with the GM. Remember, these methods should be of about average difficulty, with small payoffs. Characters, including other PCs, may seek out the new angel for their own reasons, and it's up to him whether or not he accepts them as servants. He may receive visits from Archangels, offering small boons in exchange for letting his servants alone!
In closing, a note to GMs: all in all, life is transitory -- but since the passing of these angels tends to come more quickly, make sure they shine all the more brightly.