Well, WML is not SQL, SQL syntax makes no sense in this case.

Your syntax doesn't even work, because there aren't arrays of scalars, there are only arrays of containers.

$foobar[] = {'aaa','something','etc'}

would have to be

$foobar[] = {['value':'aaa'],['value':'something'],['value':'etc']}

Thus you'd need to specify the key 'value'

[variable]
name=foo
find_in=foobar
key=value
[/variable]

And now the only difference to my suggestion is, that with my syntax you'd write

[variable]
find_in=foobar
key=value
equals=$foo
[/variable]

You could be a bit more specific, I need to guess here pretty much. Do you mean this?

[if]
[variable]
find_in=my_array
key=x
equals=1
[/variable]
[/if]

Well, that might not be the best syntax, but I can't think of something better.