It's true that the rule of thumb only requires the hyphen for the adjective, but nevertheless, as a noun it makes more sense to me with a hyphen too.
When I try to think of analogous compounds formed from present participles, in fact many of them seem even more closely connected, written as one word without even a hyphen. Birdwatching, babysitting, dogwalking, windsurfing,
If there's no hyphen at all, the noun might appear to be the subject of the verb. For instance, 'dog walking' could be a dog that is walking. It might depend on each individual case how closely the two words are felt to be associated.
It's an interesting question, though any longer discussion of the general principle would probably need to be in the Sprachlabor instead of here.
In any case, I support the suggestion to add the hyphenated spelling, and I might even be inclined to list it first.
And the implicit question about whether the search algorithm can find both words if they're entered with a hyphen between them may also need dealing with more globally. It seems similar to the problem that Gibson mentioned here:Siehe auch: compound adjectives