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
• HSS
• 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-565C
• YN-468C (II)
• YN-467C (II)
• YN-465C

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.

See:

• http://flashhavoc.com/yongnuo-yne3-rx-receiver-announced/
• http://flashhavoc.com/yongnuo-yne3-rx-receiver-released/

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.