I'm playing with TLINE and noticed it does not seem to care about FILLP (unlike LINE function). I'm interested what is the reasoning behind this? Would it not make sense for all draw functions to support fill pattern if it is supposed to be programmable hardware bit mask or some such?

it does - see manual how to enable fillp for sprites/tline:
https://www.lexaloffle.com/dl/docs/pico-8_manual.html#FILLP

Ah thanks! Weird, can't see how I missed that. Feels like disappearing socks.

Sprite functions might be added to the list in the first paragraph of FILLP() too (they were not listed). As for search Google's slop generator convincingly claimed that tline is not supported.