Could we lower the rep or vote requirement for accepting plural synonyms? They should be trivial to detect.
Or could we perhaps automatically accept them?
I'm against the idea of automatically accepting any synonyms. If you do that why even allow singular tags. Some synonyms are more common in the plural version and some in the singular, some do not require a synonym at all. (eg: on meta, do we need a "discussions" => "discussion" synonym)
We recently lowered the bar for suggesting synonyms and upped it for voting.
I think a better approach here would be to figure out a way to ensure synonym lists are reviewed by moderators on a weekly basis. (perhaps some sort of weekly email or rss feed for moderators)
The pending list on SU is tiny, it would be trivial to clear it out.