Did you ever end up solving this?
I was hoping to add some accessibilityLabels onto my complications, but I'm encountering the same issue.
It feels awfully shortsighted of Apple if this isn't fixable, as it would literally make VoiceOver on the Gauge complications only suitable for weather apps. 😕
I would also make the argument that all the Templates have this issue, though not as bad.
For instance, if I make an older CLKComplicationTemplateModularSmallRingText, I can provide a decent accessibilityLabel to the Text Provider, containing as much detail as I want... but VoiceOver is still going to add "X Percent" (describing the Ring) afterward, even though I'd like to provide a more contextual description myself.
I have no idea how many people use VoiceOver on Apple Watch, and I realize the likelihood of anyone actually encountering this bug is very low... but it still feels bad to not have a better solution.