Hoyas are listed as non-toxic to dogs and cats, and growers have reported experience with their dogs, cats, and birds eating them without any negative reaction. I myself have seen my cat chew on vines without incident (other than my ire). He is attracted to the ones that have a grassier texture, especially retusa, but will ignore most species. So maybe avoid the Hoyas that look like the types of vegetation your lizard might forage on in the wild. Here is a thread where people give some examples of their experiences with different pets.

It is true that some varieties have been found toxic to sheep and cattle, but sheep are notorious for making themselves sick by eating too much of even normal foods. So, I think it's more of a gorging than nibbling situation. Probably avoid Hoya australis, just to be on the safe side, though.

It's possible a Hoya may cause irritation in some animals or individuals, in large doses, but it's no more poisonous than any other plant you might put in the cage. If it were me, I would try it and just keep an eye on him in the beginning to make sure he doesn't go nuts on it.

I was easily able to find several lists specifically for reptiles that said Hoyas were safe in terrariums:

LLL Reptile

Reptile Forums.co

Lizard Lounge

Hi greedyghost thanks for the
quick reply! That will help a lot
I have dogs but they have no
Interest in any plants, other than
To pee on . All the hoyas are up
Off the ground!! I'm having a good
Time lurking reading old posts
on several forums. Each one seems
To have its own tone. The hoya
people all really know their stuff !
Thanks again.
Linda in Texas

No problem! We all want to turn over every stone, to protect our pets and animals. :)

Glad to hear from a (former) lurker. I agree that the different Hoya forums all have very different tones. It's amazing how unspoken rules and cultures evolve online.