Good news - my WRAPs now pxeboot OpenBSD as expected! The culprit was not pxeboot, but the etherboot PXE code 5.3.12 in BIOS 1.08 and 1.10, as supplied by PCengines.
After building an etherboot 5.4.1 binary on rom-o-matic.org, merging it into the BIOS and flashing the WRAPs, network boot of OpenBSD now works very nicely :-) I'll document that a bit more in detail in order to wrap up this thread, and to inform PCengines that OpenBSD can easily network boot. Thanks to all who replied, notably to Marc who triggered me to try replacing the etherboot code in the BIOS, Rolf
