Hi Kevin
On my Windows they're in
C:\Program Files\Calibre2 (64bit) and
C:\Program Files (x86)\Calibre2 (32bit) and
C:\Program Files Portable\Calibre Portable 2.9\Calibre and
D:\_Sandpit\Calibre Portable 1.48\Calibre
I believe some calibre users install Windows portable on a cloud drive, eg Dropbox, and some will have it on a thumb drive. So... it could get a new location (drive letter) depending on which computer they're using.
I suggest the location be configurable and have it persist.
My Mint box is inaccessible (up at the farm), but I know its not in the 'standard location'. So... again I would suggest the location be configurable and have it persist; although I doubt I'll ever use Sigil on that box.
That said, my thumbs up didn't indicate I was anxious for such a PI, just that your idea seemed more sensible than creating an Output PI for the 'proprietary' HTMLZ format, which is readily and freely available 'next door'.
Someone would inevitably ask, "Oi, wot about Output PIs for PDF and RB!"
BR