Only, it turns out that you can, if you just know how:
Basically, it's yet another back-end validation issue: while the upvote button is missing from the page HTML, the SE back-end is still happy to accept and display upvotes for election comments, so all you need to do to restore the missing functionality is to edit the page HTML in your browser's dev tools and paste in the code for the upvote button from some other page.
What I'm not sure about is whether the actual bug is that we can upvote election comments like this, or that we (supposedly) cannot. Specifically, while the bug report above (about the inability to upvote election comments) is marked as status-bydesign, Anna Lear's answer to it seems to suggest that it may not be really by design (emphasis mine):
This was taken out a while back along with flagging.
Nobody seems to recall exactly what went bust there once flagging was taken out, but for the moment this is by design.
So, is it by design or not? Did somebody decide that comment votes on election nominations are bad and should not be allowed, or was it just a side effect of disabling comment flags that the devs decided not to bother fixing? Who knows?
The thing is, I'd kind of like to know, before I add the code I wrote to restore the upvote buttons into SOUP. :)