So while reading Human(ish) and working on my forthcoming review of it, I came back to an idea I've had for several years, where one would use FASERIP's Universal Table.
Essentially one would take their Skill's percentile score in BRP and use the appropriate column on the FASERIP table to determine success. In this hack you would want to roll as high as possible and in opposed tests the best result would win. Gold/Red beats Yellow/Silver and Green/Bronze. Yellow/Silver beats Green/Bronze. If both result are Green/Bronze the winner is the defender.
As an example, let's say that Gideon Hawkins has an Occult of 51, so he would roll on the Amazing column, if the roll was 25 or less he would fail. If it was 26 or more he'd get a Bronze/Green result; if it was 61 or more he'd get a Silver/Yellow result; and if the result was 91 or better he'd get a Gold/Red result. The GM could require a higher result for tougher challenges. And in a contested challenge the higher result would win.
Pretty much everything else would use your favorite version of BRP/Call of Cthulhu/OGL d100 system.