wrote:

My goal was to try and fly as fast as possible maintaining an altitude
very close to 18k by speeding up in lift and slowing down in light
lift or sink.

It's perfectly fine to do this for fun, and I'm sure you know that this
"Anti-McCready" tactic is the most inefficient possible.

noticed my Becker transponder registering 18.2k which is, of course,
a no no.

Transponder altitude is measured based on the standard atmosphere
pressure setting (QNE) and is not related at all to the 18'000 ft
ceiling which is based on the actual atmosphere pressure setting (QNH)
to which your altimeter should have been set.

So what to do? If I pulled the spoilers at that speed, the shock might
destroy them or the glider.

I've read further down that you flew a Sparrowhawk which I don't know.
But every glider I do know (all of them JAR certified European gliders)
allow for spoiler operation up to Vne. Sure, if you just yank those
spoilers fully open at Vne, you *will* break or at least bend something,
but if you hold the handle with a firm grip and operate them carefully,
then you're perfectly fine. After all, those spoilers are *designed* to
limit the dive speed at Vne! Of course, verify this with your POH first.

This story raises some questions about VNE at various altitudes
which should be of interest to all of us glider pilots. I Googled

You shouldn't need to Google, it should be right in your POH. Again, I
don't know the Sparrowhawk, but all the glider POHs I've seen so far
list Vne vs altitude. I usually write down these numbers and stick them
somwhere onto the instrument panel.