Almost all 1/2 lenses cover on a GH1 too. But you have to set the 2x digital zoom mode - which is about the same quality as the GH2's ETC mode give or take.
In both cases however you end up losing the a wide advantage of the lens as well as the 12 or 16mpx resolution of the respective cameras. You do still get to keep the speed advantage tho - which is nice on a lens like my Fujinon 12.5 - 75mm f/1.2
The worst part of these lenses is that there are just about none for a reasonable price which will fit properly in any of the available C -> µ4/3 adapters. Almost all C-Mount lenses have to be irreversibly modified by shaving down their mount surfaces and etc. All C adapters for the µ4/3 mount have a lip 36mm in diameter in order to inset the lens's mount surface INSIDE the µ4/3 mount throat - which is about 38mm in diameter when you account for the 3 bayonet keyways. This means that you have to find a lens which has a mount surface less than 36mm in diameter and that limits the selection to almost nothing! My fujinon for example is 53mm and that's about average. The only way to get around this without invasive surgery is to use a teleconverter and doing so loses the speed advantage making many other 135 options much better choices!
In all is it worth it? No! Absolutely not! Not with the GH2 and now the OMD which have great high ISO performance curves! C-Mount lenses WERE advantageous when such speeds as mentioned above were needed due to the fact that the GH1 and other 1st wave µ4/3 cameras sucked nuggets at ISO 800 and above. I suppose it could still be worth the trouble if you're getting the lenses for free or if you actually like engineering modifications. I'll also add that it's nonsense to recommend staying away from older lenses due to IQ inferiority - It's just not the case. Anything from the early 70's on is going to be on par with most modern(ish) lenses given a relatively similar price range.
Instead I would rather recommend finding a parfocal zoom with a constant aperture in 135 format. Or at least one where the aperture curve is a sharp falloff at the long end. Unfortunately there are no databases of such information so one is left to buying & trying along with the reading and hunting of hundreds of spec-sheets. I can attest to this personally with now well over 100 different 135 zoom lenses. (Wanna buy some off me? Anyone? ) The best I found on a budget like you mention are the two MF Tamron SP lenses which sport very impressive MFT curves! The SP 28~80mmF/3.5-4.2 (27A) and the SP 35~80mmF/2.8-3.8 (01A). There are others as well; even some very nice AF lenses made for other mounts - which of course do not AF on a µ4/3 camera.
The use of one of those and the increase of your ISO setting when needed (along with a good NR plugin in AE or Fusion) will produce much better end results than you'll get with any C-mount lens much under $1k. Annnnnd, of course none of these options do anything for your shaking... err, anti-shaking. MF lenses on OLY µ4/3 bodies can't use the IBIS during video and there is no IBIS at all on Panasonic bodies. So if you also need AS of some kind you're optioned out and limited to the kit-solutions! The 14-140 or the 14-150 would be my first choices!!! With the 14-150 selling for $400 used and the 14-140 selling for $500 used they're not too far out of your budget. [I think the op specified a budget of $300 or something...] Stop smoking for a month and you've got the difference already.
Maybe it fits here too?
OTOH, I have a 4.8mm F/1.8 CCTV lens which is kinda fun...
MF lenses on OLY µ4/3 bodies can't use the IBIS during video and there is no IBIS at all on Panasonic bodies. So if you also need AS of some kind you're optioned out and limited to the kit-solutions! The 14-140 or the 14-150 would be my first choices!!! With the 14-150 selling for $400 used and the 14-140 selling for $500 used they're not too far out of your budget. [I think the op specified a budget of $300 or something...] Stop smoking for a month and you've got the difference already.