As I said, you're allowed a SINGLE copy of the OS to be installed on a SINGLE computer. So if that installation is on your MBP, then running it on your iMac as well would be a violation of the license agreement. The iMac has a license to run Lion and Lion only; on the MBP you could run both without running afoul of the EULA. Now, if you went out and bought a retail disc for 10.6, that would allow you to install 10.6 on the iMac, assuming that the hardware support were there. Which is why I'm going to pass on telling you how to get around anything. You'd just run off and try it, wasting time and money in the process.
Just as some people need to realize that they won't be able to run XP on their shiny new computers (not that they'd want to if they understood some of the major architectural changes to Windows, but that's neither here nor there), Mac users have to come to grips with the fact that you can't necessarily just go slapping old copies of Mac OS X on new units.
If this or that program doesn't work on Lion, then you will need to make some allowances based on the importance of using those programs. It's just how it is. For better or worse, Apple has always been a company that's willing to kick its user base in the teeth with a steel toed boot at Christmas if it makes life easier for Apple in some way. Just take a look at the big brouhaha over Final Cut Pro X, and the fact that it's looking more and more likely the Mac Pro is going to join the MacBook in the annals of history. Pretty soon Apple will probably just get out of the computer business completely, since they already get 60-70% of their revenues from iOS devices. Not trying to be overly harsh, just laying out some things so you can start developing some long term contingency plans.