There are studies which presents both positive and any effect of vegetation cover on meiofauna abundance/ diversity in sediment. However, you need to remember about some individuals inhabiting the leaves system (especially Harpacticoida) - that could cause higher abundances and diversity at vegetated bottom in compare to bare one. My personal observations (study from the Southern Baltic Sea) shows that there are no significant differences in meiofauna abundances and number of species between seagrass meadows and bare sandy bottom.

I've been recently involved in a study of the associated macrofauna from Saccorhiza polychides holdfasts and I observed an impressive abundance and diversity of macrofauna and also meiofauna (which was not studied here)!