Well, this is not a solution per say, but one workaround that pops right out my head is string concatenation .
Something like:
{{ $trailingSlash := "/" }}
{{ $newBaseURL := $trailingSlash | printf "%s%s" $Site.BaseURL }}
and use the {{ $newBaseURL }}
instead? Yeah, its a duct-tape approach. But duct-tapes are easy to work with!
I am very new to HUGO too, so my solution may not be the most reliable