What does it mean that NiMH batteries do not have a "float voltage"?
As long as the charge voltage is below whatever the "fully charged" battery voltage is, presumably you should be ok.

The problems with NiMHs is that continuous charging can lead to a build up of hydrogen gas and a rapid temperature rise but this is when the charger keeps trying to pump energy into an already full battery.

Dropping the charge voltage below this threshold means you'll lose a little capacity (as it'll never quite fully charge), but now the battery can never be overcharged. The only thing I'm not sure about is any potential lifespan issues from having an incomplete charge cycle, you'll have to wait for someone with more in-depth knowledge of battery chemistry for that one.