Not sure what counts as headerless but no CLUT (colour look up table, think palette on most other/2d systems) is more a different matter from where I sit as that is more fundamental to the experience than some header that was maybe not even used by the game to do something useful and more there to appease the neurosis of various devs that were told to always have such things in school.
Afraid I have no specific PS1/TIM knowledge on how this gets handled. Generally if a tool expects something like that I might fake having a header (can be as simple as padding out the start to the relevant size, though might have to be somewhat legit too even if copied from another "valid" file). If you have to make a CLUT then first see if you can find it elsewhere in the files (it might be a separate thing, sometimes devs do this to simplify reads of things rather than having to fetch multiple files multiple times) but you might also see what goes as far as ripping it from memory if you can get the relevant file loaded up within the game*
*might also be worth trying overwriting something that appears near the start of the game/in the title cards and seeing what happens there in case the CLUT is just somewhere else in the file.
