I’ve answered my own question, unsatisfactorily, but well enough that I can proceed. It’s unsatisfactory because I will be without that good scimitar.
I restored a save to a point where the portal room was still blacked out. I entered using the BD Guide lever code – which I had not done the first time I entered – and the portal appeared.
The portal’s appearance had nothing to do with Tax. He is present and the portal is present.
The portal’s appearance may have had something to do with the manner in which I switched the levers. Or its initial failure to appear may have been a lapse in video card function (two NVidia 6800 Ultra's in SLI). Or it may have been something else.
***SPOILER***
With the portal present, I have not yet killed the illusionist in the hallway north of the portal room. I’ll be careful to save before running my cursor over the illusionist’s chest. Maybe by a series of reloads I can regain the scimitar.
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