Stacking an ST-E3-RT on top of a YN-622 transceiver could work, but you'd only have ratio control in eTTL over any pre-MkII Canon EX speedlites connected to YN-622Cs, not manual power level control, and you're still using two separate triggering systems.
A better solution might be to wait and see how the YNE3-RX receivers work out. Right now, there are some unknowns about them. And they're so new that there aren't a lot of field reports right now. But the assumption is that these will allow full power control over the MkII EX speedlites (and other Canon camera menu-commandable flashes) via an RT master (such as a 600EX-RT, YN-600EX-RT, ST-E3-RT, or YN-E3-RT).
The supported features that are listed are:
- eTTL, M, and MULTI modes
- remote shutter release
- PC output for manual-only sync (studio strobes, manual flashes)
- USB port for firmware upgrades
- compatibility with single-contact triggering (manual-only or other-brand flashes)
This is a pretty sparse description. We're assuming remote power control and most of the 622 features set.
However, note that on the promotional announcement the YNE3-RX, that only the following flash models are listed as "compatible" (by which we're assuming hotshoe communication/TTL-type compatible) are:
- Canon 600EX-RT
- 580EX II
- 430EX II
- YN-568EX (II) [which probably means either the Mk I or II]
- YN-468C (II)
- YN-467C (II)
Presumption would be that the MkI Canon speedlights would have a similar behavior to that on the YN-622 triggers: only ratio power control in eTTL mode, or possibly manual power level control if the YN-E3-RT dedicated transmitter unit is given a backwards compatibility mode (like the YN-622-TX's) to use eTTL settings to "fake" manual power levels.
Also, a post from a YN engineer on the POTN board indicates that Supersync is not possible in the RT system.
So, there are a lot of unknowns, and we just have to wait for actual field reports to let us know for sure what's actually going on.
And keep an eye out on the flashhavoc.com blog itself, and probably the photographyonthe.net thread on the YN-600EX-RT/YN-E3-RT (where elv--the author of flashhavoc posts) for more updated information as it unfolds.