As far as I'm aware (and I may not be fully aware), you can get the path to the binary (the plugin folder) and the path to the data folder. Both are in libobs/obs.h
Code:
const char* obs_data_path = obs_get_module_data_path(obs_current_module()); //path to data
const char* obs_bin_path = obs_get_module_binary_path(obs_current_module()); //path to binary
obs_current_module() gets the current plugin (aka module)
obs_data_path will then be something like C:\Program Files\obs-studio\data\obs-plugins\plugin-name
obs_bin_path will then be something like C:\Program Files\obs-studio\obs-plugins\32bit (I assume, I've never used this one)
I'm not sure if it consistently returns the absolute path or relative path, you may have to write a routine to check.
It may not be ideal for your needs because, if memory serves (and it doesn't always), Windows doesn't like you writing to C:\Program Files (running OBS as admin gets around this). Evidently, if OBS is installed somewhere else this won't be a problem. I think Linux and Mac are less restrictive but I'm not sure.