A lab cache is a type of cache and the cache size (virtual but not really) is irrelevant. Being able to do a top finders of lab caches is trivial.
The problem is everything else that the Top Finders page supports. Lab caches do not have country information, so setting country options would cause problems. Similarly there is no size, difficulty, terrain or anything else that the Top Finders page supports. You could do a Top Finders world wide, but not per country.
For this reason lab caches are held in a separate database and so are not included with many of the tools. One solution (that was discussed and rejected some time ago) was to turn them into real caches - eg size virtual, D0, T0, But there is still problems with the other missing data.
In short it can't be done because a lab cache does not follow the normal geocache rules for what a geocache is.