Have you given any thought to having multi-algorithm rigs? I'd have to check if cgminer and vertminer will coexist on the same box if one of them is idle at any given time, but if so, it'd be handy to point cgminer at betarigs port X and vertminer at port Y, and then have the same rig be rentable for both.
If someone rents it for Vertcoin, it becomes unavailable for scrypt, port Y starts serving up shares, and vertminer goes to town. And likewise for scrypt.
I asked the OP if this was possible, but it seems not, and i'm not sure it could be. To show up as online your rig has to be mining, right? If it's idle, you also show as offline. And if you were to have vertminer and cgminer going at the same time, you'd have bad hashrates and crashes galore i'm sure (i don't even want to think about testing, lol).
Maybe it would be possible, but i think it would have to be super complicated. Such as, when someone rents for Scrypt, the site idles your vertminer or maybe wipes the vertminer.conf pool addresses and restarts your vertminer (so it's unable to mine), then adds customer pools to cgminer .conf and restarts cgminer. Maybe this could work, but it sounds tricky, and as you say the two miners together could be unstable. He'd also have to add API access to add the site's ability to control the miner settings/restarts and add your own pools after rental ends, etc, which complicates the configuration of everything for most people.