Time to corect myself: If I powercycle printer, it stops working... It seems I flipped power switch too fast & didn' reset printer. So - hotplug it is.
Fortunately, I can read Russian, because in Russian thread hotplugging is explained in detail. Mounted USB HDD ext3 partition on /opt, placed hotplug.sh, sihp1020.dl and usb_printerid on disc, modified post-mount & post-boot scripts, and MAGIC - printer is working!
... Untill first time I rebooted router without attached HDD...
Plugged in HDD - and no printing... My partition with hotplug, firmware & the rest is not mounted....
... Few hours later, after playing with post-mount, hotplug.sh etc. and a lot of reading later - solution: FSTAB ! Everything is working
... Few hours of reading later - flashfs on WL500g Premium is big enough to store firmware. Ouch!
Epilogue: Everything is working now
& I had a lot of fun getting to it long way round
Thanks, Oleg & everybody else.
A.