I think `tagged waterway=stream, intermittent=yes` is actually appropriate here.
To address your qualms about using these tags:
> In other words, it's a pretty
> significant local feature, but to tag
> it a stream would be a misnomer.
You are not tagging it as just a stream, but as an intermittent stream, which is different.
> I know there is an intermittent=yes
> tag [...] but I'm not sure if
> renderers or anyone else would pay
> attention.
As you correctly point out, that's not your problem, as you are not supposed to tag for the renderer.
Actually, I believe At least the main map at least Mapnik https://www.openstreetmap.org/ (i.e. openstreetmap-carto plus Mapnik) now supports `intermittent=yes`.
`intermittent=yes`, and renders it as a dashed blue line ([bug 805][1], fixed in 2015).
At any rate, the distinction between "intermittent stream" and "permanently dry" is somewhat arbitrary - even regular streets can become streams if there is enough rain :-). So I don't see a problem with using `intermittent=yes` even if the stream bed is almost always dry.
[1]: https://github.com/gravitystorm/openstreetmap-carto/issues/805
