Turns out this is a 'simple' permissions issue. The user that php-fpm runs as has no logical access to the directory that holds my files. Of course, I'm perplexed by this as it does otherwise see and serve PHP files in that tree, but that's way outside the scope of this post/forum.
Since I don't think I have access to delete my own post (mods, please do), I'll leave my original query here:
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I'm a bit at wit's end here. I tried to get help on the main ImageMagick forum, but since I'm accessing it through IMagick, I guess I'll try here, too.
Essentially, while 'convert' will pick up custom fonts (which is handled through fontconfig) from the commandline, using IMagick somehow doesn't consider those fonts. I am even placing them in a default fc directory, but running exec('fc-match <fontname>') doesn't pick them up.
I realize this is partly a fontconfig question, but is there some material difference in how IMagick or the IM API interfaces with it?