Okay @henryhbk and @johnbrooker, I’ve got good news and bad news.
Bad news: silk (I’m using 8mm Habotai, very thin) does not stick to the mat at all. Something about the texture or finish. If you stick it first to low tack masking, the masking will stick to the mat. But in that case, might as well just stick the masking to the crumb tray with magnets. Well, maybe it would prevent cut pieces from flying around if you used the mat. Yeah, that might be worthwhile.
Good news: I tried the silk twice, once just sticking it down to the crumb tray with lots of magnets, and the second time I put some low tack masking on first (helped a lot with keeping it flat) then held it down with magnets. It cut very well (had to use a little more power–10%-- to get through the silk vs the tissue) and it does appear that the edges are cauterized.
This photo shows, left to right, the masking plus silk after cutting and peeling (it was a kiss cut–the masking was not cut through); middle one is the fiddle that was peeled off the masking, and the far right one is the fiddle that was cut directly on the crumb tray with magnets holding down both interior and exterior of cut (I’ve got some teeny neodymium magnets).
Here’s a closeup:
Edit: the torn looking bit on the upper right edge is where I had to cut it with scissors because I accidentally blocked that bit of silk with a magnet when I was laser cutting. Duh!