I suggest initially talking to Mallatite (suppliers of Jerol posts) about the problem. My feeling is that a small hole drilled into the post to accommodate a grub screw is unlikely to affect it structurally, but they would be the best people to ask as they have probably come across this problem before.
A secondary post sounds an expensive solution and, as you say, being an untested arrangement it is uncertain how it would perform in a crash.
I presume you’re aware that TA 89/05 was withdrawn by HA in 2008, and Highways England now just use the UK National Annex to EN 12767 as their guidance. But there is of course nothing to stop a local authority still using it as their standard.