I think it’s been done a couple of times, I’ve done the first section from Glenfinnan over the bealach, the climb is good track for most of the way, however the descent is faint track at best for a lot of it, I remember it being frustratingly unrideable until much nearer the bottom.
From Strathan to Sourlies would be quite broken riding, there’s a very clear path but it’s the usual mix of big loose rocks, mud/water splashes and burn crossings.
Sourlies to Inverie would be largely unrideable until the top of the pass after the river Carnach, it’s a mixture of beach (if the tide’s out) or massive boulders if the tides in, followed by a big sort of tidal marsh with lots of deep narrow channels to hop.
Obviously your experiences may vary! Amazing place but one that I think would be more enjoyable as a hiking trip rather than a biking trip…
A better option might be to head straight through to Glen Kingie then further to Loch Quoich? from there you could head towards Kinlochhourne and head into Lochan nam Breac, there is a much better quality path from Glen Barrisdale which links to Lochan nam Breac, I’ve only walked some of it but it all looked rideable…