I had the same idea of repurposing a seedling mat rather than buying something new. I tried the mat by itself and it didn't get warm enough, so I bought a separate thermostat (and an independent thermometer bc I didn't want to just rely on the one on the thermostat). It seemed to be ok when set up inside our house in late summer for 2 chicks; however, the mat I had did not keep the temp very steady (if you search seedling mat, the thread should come up where I asked about it and reported back). Even with the thermostat set with a narrow range, the on/off cycle meant that there was at least a 20 degree swing between high & low temp. I didn't worry too much because mine were indoors and it was relatively warm still, so the surrounding air temp did not get below 70 even at night and I watch their behavior and they seemed content, though at first they spent most of their time in the cave. I'm sure different pads and different thermostats vary in their quality and effectiveness, but I did not trust mine for use under colder conditions, and I doubt a seedling mat itself without a thermostat will work; typically, they're only capable of increasing the temp 10-15 F above the ambient air temp. I have chicks now indoors, and I went with the heating pad this time and the temp seems much more consistent (now that I have the heat setting right, and also switched from a baking rack to a piece of curved hardware cloth with taped edges).
I agree with Ridgerunner about the benefits of setting up a cave rather than underneath like a reptile mat; the cave method means that there is a zone where the heat is trapped, which will keep them warmer than if it is just underfoot and the heat is very quickly lost to the air around them.
I also strongly recommend testing your set-up now to see if you can get consistent temperatures with your mat so that you have time to change if necessary, including set up outside to see if there's adequate heat under those conditions. You can get a relatively cheap indoor/outdoor thermometer that records the high/low temp within a 24 cycle so you don't have to constantly check. Ideally, you'll want to check the temp of the air inside the cave as well as the surface temp reading. There's a lot of discussion of what those temps should be - I don't have enough experience to give definitive numbers, but surface temp shouldn't go too high that it could burn them, and air temp below shouldn't get too low. There's a fair amount of disagreement about what the air temp in the cave should be, since it's not the same heat transfer as with a bulb - the cave primarily relies on direct contact heat transfer to the chick's back vs. convection with a bulb. At the very least, you'd want to have a sense of the range you have - how warm you can get it under those conditions so that if chick behavior shows they're too cold (huddling, crying) you know that you can increase the heat to make them comfortable.
