Like Frag, I use Photoshop. I have a bunch of support utilities that go along with it. I'm able to embedded subpalette reference directly into PCE colors on Photoshop. Since you use stepping of base 36 for R/G/B, I have a util that creates slight differences between the steps. I have a utility as well as a lookup chart (for manually editing stuffs). Each embedded color corresponds to a 0 to 15 number (aka subpalette). The other method, is just to create a palette map to overlap top of the pic/map/etc that you're working on. I use 8x8 grid blocking on the palette layer, a total of 16 colors and each color is a subpalette. Where ever the 8x8 color from the palette layer overlaps a 8x8 area of the pic underneath, that means that 'tile' or cell underneath belongs to that subpalette. I export both layers as BMPs and have util that creates, the tiles, supalettes, tilemap, etc. It also creates a few other files increase I want to bring it into a different program (like a tile editor). Etc
I do this for sprites as well, when they are made up of smaller sprites with different subpalettes.
It sounds complicated, but it's fairly easy to use. I've also used photoshop to make game maps. You can use PS as a pseudo tile map editor. Far credit due; I learned a ~lot~ of my Photoshop kung-fu from Fragmare.
Can't really help you out on the HuC pcx stuff. I don't use those functions of PCEAS (which HuC just passes along). I usually need a lot more flexibility than what it can give you. I don't even use PCEAS anymore (HuC generates ASM code and passes it along to PCEAS). I'm created a new fork (PCEAS2) with some new directives and such. One of these days, I'll add BMP support to it - with embedded color subpalette decoding option, as well as a few other options (like remove redundant tiles, etc). Not that I need it, but sometimes you just want some quick and dirty support to test things out.
Btw, is that your stuff (original)?