So I had some time yesterday to figure out the format. I only had a single skin for a Piranha ship (the only search result when I search for "UCES00465DTEAMSKIN0000") to go by, but it's actually quite easy:

1. Download the ZIP file from this forum post
2. Use psp-save to decrypt the save content (mode = 5, key = 16 bytes, first byte is 0x01, rest is 0x00 (create this file manually))
3. The decrypted file will have a 32-byte header (I don't know what its contents are, need more samples of skins for this)
4. After the 32-byte header, there's 3 image files, each image file contains 16 x 32-bit RGBA palette entries and then 128x128 4-bit palette index (the "pixel data")

Now you can just dump the pixel data to 3 separate images, edit them, then pack them again (each image must be limited to a 16-color palette but the palettes can be different for each of the 3 images). Then just re-encrypt with psp-save and it should load in the game (tested on a real PSP with the UMD release of the game, and on PPSSPP).

If you still have old Pulse custom skins on your PSP memory stick, please dust it off and attach a ZIP of
Code:
PSP/SAVEDATA/UCES00465DTEAMSKIN0000
here (if you have more folders like 0001 instead of 0000 at the end, send them too). For non-EU releases, replace UCES00465 accordingly (can be found on the spine of your UMD case). I won't have time to look into this in the next days/weeks, but if we have collected a bunch of skins, maybe there's a way to make a new skin editor by the time I can revisit this.