I've never had a crispiness problem I could attribute to Splenda -- did you try baking it a little longer? Also, I've noticed when making other stuff with Splenda that it can tend toward the grainy if you don't dissolve it in something warm . That's one of the reasons I use the warm method meringue (the other is that it's supposed to hold up better in a pastry bag). If you can't just bake the meringues a bit more -- say, if the outsides are getting too dark but the insides are still mushy -- try doing that.

The only time my meringues don't crisp right is when I make them on a very humid day -- they just don't do right. It wasn't about to rain the day you tried your Splenda meringue, was it?