We know that official RLVa support is "on the roadmap" but also not coming any time soon. I'd like to propose something that could be done
immediately
that would add a small measure of "official RLV support" that would extend the functionality without interfering with any existing stuff, leaving plenty of space for "the rest" at a future time.
Simply, the viewer could hold an official new data field, the "RLVa version" (or whatevs we want to name it), and a new scripting command llGetRLV() would cause the simulator to ping the viewer of the object's owner and get the string. It's that simple.
What this gains for us: Trivially, the Second Life Viewer can set this to "" for now, since there's no RLV(a) support. Other viewers could set this string to the appropriate version. Future scripts could find out RLV enabled status just by checking this string, with no need for the message protocol. Nothing existing today breaks/changes in any way, but suddenly all future scripts could use this instead of current methods, which tend to spam the chatboxes of anyone without RLV enabled.
The back-and-forth script dance of version checking is such that currently, many makers of RLV items simply assume that if you've worn an RLV-possible item, you knew to have RLV on. This single official step would help RLV scripters get their stuff working, and help RLV abstainers be spammed less in the future!