Yes, you can do that.
It helps to get the draw priority approx. right and I've never seen any documentation of where the Garmin maps are in the hierarchy.
MapsetToolkit offers priorities up to level 31, and transparent or not, so you can arrange which map sits on top of which. In theory, anyway.
I reckon by trial and error that Metroguide v9 sits at 28 or 29. Not 100% certain about this though.
I have added contours which sit at 31 and transparent.
So to add OSM into the mix a priority somewhere in the low 20s would seem about right. It then sits invisibly 'under' Metroguide and if you switch Metroguide off the OSM map displays, in my case still with the contours on top. (I think Andy's map has contours integrated ? so you would need a different approach in this case)
An interesting experiment would be to create a OSM TYP file that renders roads and areas invisible, leaving only paths and bridleways and cycletracks, plus a few POIs - ie just the useful bits that aren't in Metroguide. Then put this at priority 30 and transparent - should be possible to get Metroguide with the OSM cycletracks etc displayed simultaneously. Might try this when I'm bored.
P.S. I think routing in OSM has to be seen as a very long-term project. I tinker around in the map edit facility sometimes and I find mal-formed road junctions all the time. Every such junction is presumably as bad as a missing road, from the routing point of view, and will have knock-on effects all around that vicinity. Add to that, that a lot of the road classifications (primary, secondary, tertiary etc) are applied inconsistently (because of the wiki nature of the beast) and its never going to work too well IMO.