Getting the correct sense of the handcontroller RA DEC reversion switches matched to the adapter made all the difference.
Following polar alignment using the mount's built in reticule, I took a 4 minute exposure with guiding off and then a second with guiding on. It was difficult to discern the difference, without pixel peeping - this mount is superb properly aligned.
Further exposures with high speed correction increasing the rate for subsequent exposures produced no discernable change. Pixel peeping verified the same.
Increasing to 5 minutes verified slight misalignment without guiding.
Satisfied that an operable guider would be confirmed by a series of exposures I set up guiding with normal speed correction on the mount and let it run for 35 minutes.
All seven exposures lined up exactly with beautifully round stars. Will post images once processed.
The Lacerta MGEN II stand-alone guider performed flawlessly with factory settings and a Borg 50mm guide scope.
The EM200B turns 21 this year and has probably never seen an autoguider. It was designed for manual guiding. I'm sure Takahashi had good cause to use +ve ground and Hirose connectors (which are still in production), making the conversion more involved than I would have liked.
A crop of the test subject, which does nothing more than show round stars. 5 minute subs. Southern Pleiades, I think. A short video of the alignment of the first and last frames 35 minutes apart.