Another Windows workaround is to create a right-click Shell extension with a .reg file:
Create a text file with this contents:
Windows Registry Editor Version 5.00
@="\"C:\\Program Files\\RawTherapee\\5.2\\rawtherapee.exe\" \"-R\" \"%1\""
Save it as <any name>.reg.
double-click and validate for the command to be written into the Registry.
Modify the path according to your RawTherapee path. Don’t forget that quotes and backslashes must be escaped by a backslash (first version of my post was ruined by the text editor, which also considers backslashes as escapes, and converts straight quotes to left and right quotes;) I had to use HTML entities; copy-paste should work now). If you want to change the name “RawTherapee Browser”, don’t forget to change both occurrences.
PS: Still searching the proper way to pass parameters through an XnView ini file…