My suggestion is to add another mode in which 2 environment variables will be passed to every sub-build and instead of the exclusion list.
PARALLELIZATION_DEGREE
PARALLELIZATION_PART
It will let users to use their own calculation and not count on the exclusion list which is not tolerant to missing/partial results ( i.e.every added test is not excluded from any sub-job and is executed on all)

I am using this plugin to execute a home grown UI integration testing in my workplace.
every test takes from ~30 seconds to more than 5 minutes.
full un-parallelized execution takes more than 3.5 hours, breakdown to 15 parts takes 11-12 minutes, but if we add a new test that is a long one we get big time addition penalty.

but we also have a way to breakdown to files containing the parts and the jobs can load the tests from a file but I need the vars to get it, i.e. /path-to/$PARALLELIZATION_DEGREE/$PARALLELIZATION_PART.json

currently I do it with
export PARALLEL=`printenv "exclusion.txt" | grep -o -m1 -P '(\d+)' `

WDYT ?