tbh there's a way... but the effort is not worth it, i think it's too tricky just for nothing....
Anyway , just for the record it can be done in this way:
Let's keep simple, so I say we have a total of 10 regions: A,B,C,D,E,F,G,H,I,L
We want A become one way with B if we hold C.
First we need to code a useless bonus where all regions are listed. It's value is always 0, 1 region is required and only C is coded as blocker. We name this continent, let me say "a region"
Then we make A border with B, as normal region
Then we make B conditional border with A where condition is hold "a region"
Now, we know that the blocker act like we do not hold a bonus, so if we hold any region, B will border with A, but if we hold C, then the blocker activates so we do not hold the bonus anymore, then B doesn't connect with A. So A is one way B if you hold C.
But as said, too much to write (specially if you have a big map) for a so small thing.
Nobodies