AFAIK we can change partition layouts via TPT with no problems at all, and AFAIR there was an old way of TPTing and flashing a ROM in which the recovery was flashed to the /system partition, the phone booted to it and from the recovery you were supposed to install another (same) recovery to the /recovery partition and from there (after rebooting) we had to flash the ROM; and so I assume we can also flash a ROM into the /recovery partition and it should boot just fine.
Two ROMs would require twice the space so it would be necessary to use sd-ext partition for /data (but we have already got working solutions like Apps2SD, Link2SD and others so it shouldn't be a problem).
The last problem would be an actual recovery - it just wouldn't be anywhere. So I thought of a few solutions:
1. Flash it onto the /oem or any other partition and find a way to boot it (eg. from the system with an app with root).
2. Integrate it with the ROM; if recoveries are open source it should be easy to do (and I believe they are, but I couldn't find recent source for CWM though)
3. Don't bother with it. ROMs can be swaped with TPT, and if someone would need a recovery they would need to flash it back to it's partition.
Do you think it's possible? If it was, it would be great! Being able to run ICS/JB on the /system and switching to GB [CM7] that is installed on /recovery (or any other ROMs) would be awesome.