Generated file paths in Tuprules in a variant build

[ffreling]

In Tuprules, you can create rules that will be evaluated in another directory with a Tupfile.
You can however reference files related to the Tuprules:

&root = .
!custom_rule = |> bin %f %o --extra-conf=&(root)/sample.conf |>

However, if you rely on a generated file in a rule, it is impossible (as far as I can tell) to reference this file in a rule.
Would it be possible to have an equivalent to &-variable for generated files? A way to specify an absolute path in a variant build.

I added a simple test case in this repository: https://code.ffreling.com/fabs/tup-recipes/src/branch/master/variant_path

Thanks.