Not implemented yet, but I think a relative search would be a good way to indicate this. So you would add a . at the start of your search to indicate that the path should be evaluated relative to the current focus.
Like this:
.//task
I think that would work pretty well. Does it seem like it would give the behavior that you want?
Also, I expect some of this might change in Bike 2, but these are good enough stopgap solutions I think.