I sold Temco for a long time, and in fact they were one of the first companies to produce and sell a lot of vent free logs. At the time, which was the late 1980's they had approved them for virtually ALL fireplaces which were capable of burning wood. It was a few years before they actually made a fireplace which was for vent free only - that unit did not have a damper in it, but was built very similar to the other units.
One well known company, healtilator, bucked the trend and never produced vent free logs and then, in fact, labeled their fireplaces not to allow them. This was not done 100% for safety, but I think they figured "what's in it for me??....in other words, they took a stand against vent-free and simply wanted nothing to do with them!
Let me summarize this way:
1. If Temco were here today, they would probably tell you in writing and in the manual that you could install vent free in that firebox.....because their manuals for their vent free logs said you could install in most any approved wood unit.
2. The lower BTU of that vent free log is not likely to overheat the walls or other parts of the firebox - consider that UL testing probably loads firebrands (small oven-dry wood) in there and may get to 100,000 or more BTU/Hour.......prefab fireplaces are, in general, designed so that the hotter they get, the cooler they get (more airflow around them).
3. If you use any gas logs, do not use glass doors closed or partially closed.
Some other ideas may be of note. One of the dangers of vent -free was the amount of heat placed on the fireplace face above the fireplace opening - so if you have wood very close AND the damper is fully or partially closed, this can overheat the wood. Mantel protectors were often suggested with vent free.
Back when I sold vent free logs, some customers asked for us to actually close off their damper areas so the fireplace would more closely match the vent-free firebox and heat would flow out the front as opposed to build up in the upper part of the firebox. We did this mostly with masonry fireplaces, I would not suggest it with prefabs as part of the cooling mechanisms (air flow intakes, etc.) might be up there.
It was also allowed for dampers to be adjusted open or closed or even cracked slightly open......some log set instructions mentioned this.
You would still need a permit in most areas to install a gas log set - after all, you are piping gas. That's plumbing! We always needed on in NJ where I lived.
Personally, from afar, I think it would work fine. But there may be a better solutions. There used to be a type of log which was called "partially vented", and although they stopped using that name, the type of sets are still available. An example is the Monesson Duzy. These are radiant log sets which throw out a nice amount of heat...and some of them allow the damper to be closed somewhat (see instructions for various sets).
With any of these sets...and, in fact, even with a wood fire the largest dangers occur when they are run 24/7 - maybe a power failure and people try to use their fireplace to keep the house warm! Other than that, the other danger with pre-fabs is using a glass door other than the one made by the manufacturer (or a listed equiv.).
Hope that all doesn't confuse you too much....