Yea, do try it out and let me know. I'll also try it out on when I get the chance to. But either way, not sure that this is a viable solution; one wouldn't want to arm LNAV while they're only being vectored on a HDG SEL. Would cause a huge mess if during such a phase, with high workload, they inadvertently arm and forget to disarm LNAV.
But more importantly, is there no reference for such thing in the manuals?