The road to the Col du Larche has been under construction for years and they won't let bikes up the road, probably because it's so steep and narrow that bikes would slow everything down too much. FWIW, I saw several bikes heading DOWN the road on both sides of the construction, so maybe they let you through if you are speedily descending?
There is a navette out of Barcelonette that will take you to the Col, but bikes may be a problem and the bus doesn't start running until early July. We didn't have any luck with private taxis, but maybe if you book well in advance you'll do better? Check in with the Ubaye Tourist Office for all these options: ubaye.com
There is a scenic little cafe at Meyronnes after you rejoin the road past the narrowest, steepest section, and a mountain hut at the col.
While it's a lovely valley with great wildflowers and a rowdy little creek, you wouldn't choose this trail for the quality of the riding. The valley is steep-sided with erosive geology, which should tell you something about what you are getting into: expect a fair bit of pushing and a little bit of scrambling. But it's all manageable and not too burly, even with a bikepacking load.
The Ubayette river floods regularly, so there are no bridges. Meaning that you don't want to even try this if the water is high. There's a fat pipe at the first of three crossings that you can use if you are nimble, but after that you'll be tossing your shoes over and wading the bike across. After each crossing, the trail gets fainter until it becomes little more than a fisherman's path.
So why bother? Because if you want to ride the multi-day Grande Traversée L'Alpes-Provence, and you don't want to work out the tough logistics from Cuneo on the Italian side of the trailhead at Col du Larche then you have to come up from the French side, and the road is currently closed to bike traffic. Even when it isn't, the Col is a non-toll road so trucks are all over this tiny, narrow, steep mountain goat path. In fact, you might want to select this trail even if you have a choice to ride up the road at some point.