I haven't played with OpenWRT SQM for a while, but if it's easy to reproduce with gfblip, it might be a simple matter of telling them your exact SQM settings and the URL to try.
Chances are changing the SQM backend (eg. between fq-codel and cake) will at least change the behaviour and likely make the bug go away.
Thanks! For posterity: I was previously using cake/piece_of_cake.qos. Changing to fq_codel/simple.qos made the blips go away, and seemingly does a better job of eliminating bufferbloat to boot.
fq_codel/simplest.qos made the blips worse instead of better. fq_codel/simplest_tbf.qos reduced but did not eliminate the blips.
Chances are changing the SQM backend (eg. between fq-codel and cake) will at least change the behaviour and likely make the bug go away.