I'm sorry I can't be of help, I have another related question that maybe you can answer me, how does a gravel vac work with sand? Won't it just suck up all the sand too since it's so light? I am building a 150 gallon to put bass and sunfish in, and was thinking about doing the same thing you are, except I think taht maybe it'll be a crappy deal once I start trying to clean it, what do you think?
Honestly, sand will not hold plants permanently or even temporarily. Its already hard enough trying to keep plants dug into gravel with Ps in it, and sand being lighter. Plus when cleaning sand, vacumming never worked for me before. I usually just do a floor sweep with a nett to p/u waste and crap along the top of the sand.
What I was thinking of doing is getting those little peat plate, and poking holes in it, sticking the roots of the plant through it and using those plant weights you wrap around the roots of the plant on the bottom side. Then add the sand over it. I think it should work. I know I said a lotta confusing stuff but heres a link to that peat plate for a better idea