https://github.com/aurelienpierreeng/ansel there's also a fork to play with

I can see that it would not work well for cases like painting over parts of the image, which Lightroom et al. allow with ease. If you try to be “holistic” in your raw treatment and like me at most do a graduated filter or mask by colour, RT works well enough (the latest versions improved it a lot, too).

I do a lot of mild re-lighting. If RawTherapee had the path masks from darktable I'd be more than happy.

One way could be exporting lossless 16-bit TIFF or PNG from RT, in some wide colour space and without sharpening, and then doing relighting + final processing steps in another tool… However, if you want to apply a CLUT after relighting then yeah, tricky.

It would be great if RT supported something along the lines of “take the entire canvas after initial raw processing but before CLUT and final touches and put it into a temporary file; allow me to do something with it in any other tool; then load it back and apply the rest of the steps on top of that”, but that might require it to store that edited canvas somewhere in addition to .pp3 file.

Can't stand roundtrips anymore. Yup, I actually apply a LUT after tone mapping in darktable.

Well, it’s not a roundtrip if you just feed the output to another tool. My processing script typically pushes RT-prepared PNG to something like ffmpeg which may do resizing and more, depending on the end format.

ART (Another RawTherapee) has a more Lightroom-like approach to masking that you might like better.