You may well be able to convert the preway to contain a ventless gas log set. The unit was made for wood combustion, and certainly would have no problems with the temperatures. If a vent is attached (chimney), the flue gas temperatures will be low. Your local codes may stipulate use of double wall pipe regardless of the temperature, but it should be an easy matter to isolate single wall pipe from combustibles and vent the unit. Also, you could insert a much smaller diameter flue pipe inside the 8-inch Preway pipes. The gas unit will not need more than 4-6 inch pipe to vent.

The problem would be if you sold the house in the future and someone had the bright idea of burning wood again. Then the installation could be unsafe. The preway will provide some radiant heat from a gas log set, but will not be a good sole source of heat.

Here is an article on indoor air quality and ventless fireplaces here, that suggests air quality may not be too bad, but makes precautionary recommendations: http://www.consumerreports.org/main/content/display_report.jsp?WebLogicSession=QZuaV3G1EMxkAUKggCjuquKDXF2loqxEFt4UV4BxocrRmehwHhmk|261379558010968802/169937909/6/7005/7005/7002/7002/7005/-1|-4674297013624850435/169937904/6/7005/7005/7002/7002/7005/-1&FOLDER%3C%3Efolder_id=463483&ASSORTMENT%3C%3East_id=333143&bmUID=1100716631605

I have no personal experience with such a retrofit, and recommend that you talk with a fireplace shop and your code enfocement department about your ideas.