Re: hitsorical record
"NGC 6537 looks very like a cross - and many other planetary nebula exhibit the hourglass appearance that could be interpreted as a cross. The helix nebula is the size of the full moon and 2.5 light years across and may be a candidate - not sure how fast its expanding!
A nearby one may have dispersed by now, or possibly have blown itself away with a greater explosion later on - maybe the crab nebula was planetary before it went supernova?"
The Red Spider (NGC 6537) is pretty unique in shape, and at 1.5 arcminutes is not very big (and would have been smaller in the past). The hourglass-type side-on nebulae like the Dumbbell are not that cruciform, and though the Helix is the size of the full moon, it is very difficult to spot, even through my 8" scope, as its surface brightness is very low. This is a key problem: high surface brightness planetaries are small, one big enough to resolve by eye have very low surface brightness.
If the white dwarf at the centre of a planetary is part of a binary (spotted one such system last year), it could go supernova (Type Ia), otherwise this is unlikely (not enough mass). There is no indication that the Crab pulsar is part of a binary, I think. There may be a supernova remnant as yet undiscovered, of course. Supernovae embedded in a star forming region could generate strange light echoes on the surrounding dust and gas.
Finally, the "red crucifix" in the sky may have been atmospheric, rather than deep sky. A curious illumination of clouds after sunset, noctilucent clouds in a strange formation, auroras, or a bright meteor which exploded (and form cruciform patterns). And finally, if your king or local lord had stated he saw a red cross in the sky (after imbibing some bad mead, maybe) stating you could not see it might be a terminal career move ;-)