On the plurals page, you can see that single-syllable en-words get -ar or -er in plural, with -ar being more common. This is probably how smörgåsar has formed plurals due to the -gås part being a one-syllable word.
Smörgåsar is kind of irregular in that it's regular. Formed with the word gås having the irregular plural gäss, smörgåsar however has become a regular en-word getting the common plural -ar.
Smörgås has one syllable? I'm hearing two: http://www.forvo.com/word/sm%C3%B6rg%C3%A5s/#sv
Is there some etymological reason linking smörgås to gås? The gås -> gåsar makes sense, but if I tried to generalize the rule "if the ending syllable is a real word on its own, then pluralize it as if it's that word" then that would only end in chaos and tears.
Ultimately, it's just an irregular, and I should just memorize it?
It’s true that the a compound is generally declined as the last element of it, however, the ”goose” meaning has sort of become obscured over time so it has lost its original irregular plural form. The etymology is unclear, but one hypothesis is that the ”goose”-part refers to blobs floating in the butter after you’ve churned it that you used to put on your sandwiches.