Well, maybe this isn't supposed to work or isn't documented for some reason.

But it works for me.

Do a search on "wendy=5" or "wendy!=0" etc.
The numeric value is the bitmask of the wendy flags, which you can figure out by looking at your config.ini file.

Unfortunately, since there are no bitwise ANDing or ORing operators in the search function, this is of marginal utility.
But I have used it to see which songs have any Wendy flag set, or which songs have one and only one Wendy flag set.

I sent a request for this to be added to the FAQ. Since it wasn't, I have to assume this isn't supposed to be a publicly supported feature.

But it is useful for me nonetheless.