Thanks, this was very helpful.
For anybody needing to do this in the future, here is what will work inside a wappler custom query:
I found you can use the mysql find_in_set function in lieu of the IN clause. It should be noted that this shouldn’t be used on large sets as the performance will degrade.
find_in_set(cast(distro_tags.tag as char), :P4)
In my example this adds a WHERE condition requiring the tag field from the distro_tags table to be one of the entries in the P4 parameter, which is a string representation of a numeric array.
The parameter can be passed directly from the $_GET variable into the sql statement since it natively has no white space. If creating your own, the format is value followed by a comma without any white space (or it will break.)
Solved!