That's so weird that you're like 1/3rd the emperor in that scenario. As france can you offer to repeal imperial reforms as a concession? or perhaps if you release austria as a french vassal then attack someone else? I'm curious what happens with the emperor as a vassal when an election comes up, or if he revokes while himself being a vassal.
No, I cannot offer to Revoke a reform - either as a direct annexer of the HRE Emperor, or as the overlord of the HRE emperor in a war in which the HRE emperor is a participant.
Makes me wonder WTF would happen if you added france to the HRE and revoked while a vassal of france.
Haha now that is a good thought!

Sadly nothing too exciting happens:
- 1579 start as Austria
- Passed all reforms upto and including Erbkaisertum (hereditary Emperor)
- Tag switch to Bohemia
- Annexed Austria, and Released as Vassal
- Tag switch back to Austria
- Enact Revoke
- Austria gets all the privileged vassals, including Bohemia
- So Austria is now an independent nation, with Bohemia (and all the rest) as vassal: they have swapped places.
What was slightly more interesting was when I loaded a save as Bohemia, at the point of step 4 of the above list - so I am Bohemia, Austria is my vassal - forgetting that Austria still had 100 IA. They of course immediately implemented Revoke (so yes, a subject nation HRE Emperor will pass reforms.)
As Bohemia, I was given the choice of whether to accept vassalisation under Austria, or decline; I chose the latter, which would normally put me at war with the Emperor I think, but in this case just meant Austria got a bunch of claims, a relations malus, but Austria remained my vassal. So I was Bohemia with one vassal, Austria, and Austria had all of its 43 privileged vassals. That's actually a good position to be in for me - at that stage in the game, I don't want all those vassals myself, because I can't be emperor and therefore I can't get them privileged. But if my vassal has them, then they will be privileged for them, and I will still have control of them because any war I start will be joined by Austria and then joined by all its vassals.
But it might be tricky to achieve that in the normal course of the game. It would be easy enough (well, possible enough) to vassalise Austria when it has passed all reforms except Revoke. But once vassalised, it's going to have trouble getting enough IA to pass Revoke; it will lack its normal diplomatic options for building IA. That said, maybe you can help it out. If you declare and win wars that naturally call in the Emperor, then probably he gains IA based on the result.
So yes, perhaps one could manage and benefit the HRE at one step removed. You don't get any of the direct modifier benefits - increased DipRep, Tax revenue etc - but you could potentially control the same number of vassal armies, by virtue of controlling the HRE emperor who controls them. It could thus be a way for a non-eligible nation - Muslim or whatever - to take charge of the HRE and get many of its benefits.
It would certainly be fun to try
(Actually, while I was doing some testing as Bohemia-with-Austria-Emperor-as-vassal, the IA went up from 0 to 6; some action occurred naturally that gave him some. So yes one could almost certainly get enough IA eventually for the AI to pass Revoke.)