http://rawpedia.rawtherapee.com/Color_Management#Output_Profile
Here is described the list of bundled output profiles, which include a “RT_Large*” set that the docs describe as “similar to ProPhoto”. I just inspected RT_Large.icc from the RT source tree, compared it with Elle Stone’s LargeRGB-elle-V4-g10.icc, which is constructed from the ProPhoto specification, and the primaries are identical in the two.
I’m guessing you want to save a large-bitdepth TIFF for import in another program… ?? If so, RT_Large will work just fine for your intent.