Quad QHD-4 Axis Disc Brakes stuck (can't push open)
You pretty much have nothing to lose by disassembling the calipers and attempting to free the pistons. They are useless now, so if you break something it does not matter.

You may try contacting the manufacturer (Hayes?) and asking if they have a service manual.

The silver cap with the Torx socket does seem to be the way into the internals. If you can measure the point-point distance with some accuracy (1/10 mm) you can look up what size it is. The Wikipdia page on Torx drivers lists the sizes. If not, take a caliper to the tool store and try different sizes until you get the  right one.

It's the inboard piston that is stuck so if you can take the outboard one out you should be able to carefully drift the piston back in with a short length of wooden dowel or brass rod (old door hinge pins work well) and a hammer.

I could not find any info on these particular brakes online but you could look for other videos about how bicycle disc brakes work so you know what to expect when you get in there. Park Tool posted one recently. It's informative, if a bit silly at the beginning.