Revisiting the old LOF (lack of feature) for the custom encoders in foobar. LOF continues to exist in 0.9.4.1.

Repro:
Make custom encoder like:
encoder: C:\Program Files\ogg\oggenc2.exe
extension: og2 (yes use og2 because otherwise foobar will blindly write tags)
parameters: -c GROUPING="$if3($meta(content group),$meta(contentgroupdescription),$meta(contentgroup),$meta(grouping))" -p1 -q7 - -o %d
format: lossy
highest bps: 24

Save.
Choose a file.
Convert it with this custom encoder.
Rename the extension to OGG
Drag it into Foobar
See the tag for GROUPING. It will be something like:
"$if3($meta(content group),$meta(contentgroupdescription),$meta(contentgroup),$meta(grouping))"

It seems that the codebase for handing *parameters* supports many of the %xxxx% but non of the rest of the TITLEFORMAT syntax.