TSC Editing is cross-platform. Open Noxid's standalone TSC editor (it's in Java btw, so you need to install the JRE) and go nuts. Other aspects like editing weapons and mapdata (map-related info) are platform-specific, while others such as maps and entities aren't platform specific, but rather lack a cross-platform method to edit them. PBM's are just re-badged .bmp files that have "(C)Pixel" hex-edited to the end, though there is a mod floating around the forums that switches the format to .bmp without the (C)Pixel requirement. Unlike CSPC most ports of Cave Story do not accept 24-bit PBMs, so keep it 8-bit 'kay.
Mapdata can be found at $9FD40 and $1467A0 in the doukutsu executable (Doukutsu.app\Contents\MacOS\Doukutsu). The format for these are different to both PC cave story's mapdata and each other. Keep in mind that the entire game is repeated in the exe, one half for powerpc macs and the other for intel macs, with different byte orders for each.
I can't remember where the weapon data was located.
No-one knows where the starting map, health, dir, event, etc. info are located, so you must do all this via TSC.
You can edit the name of the save file in Doukutsu.app\Contents\Info.plist and Doukutsu.app\Contents\Resources\Info.plist