According to http://sci-toys.com/ingredients/sucralose.html, splenda is
600 times as sweet as sugar. So, you'd need far less sucralose to
sweeten wine to the right degree- on the order of a gram instead of a
pound. Even assuming that sucralose and sucrose change the density of
wine to exactly the same degree (which it probably won't), a gram of it
isn't going to change the density of your wine enough to observe with a
hydrometer.

Stephen claims that splenda won't ferment. I hope nobody will try
this- it it were to be broken down by yeast or anything else, it would
give you small organic molecules with chlorines in them.
Organochlorines, with notable exceptions (including, presumably,
sucralose), are nearly universally bad- you don't want them in your
wines.