I would post it, and NOT "by written permission only". Put up NO TRESPASSING or NO HUNTING signs and make sure EVERYONE knows they aren't allowed to hunt it. The ones that ARE allowed, will have permission.
If you don't...don't complain when some slob is in your spot, or just killed your buck, or when they stomp all over your property and push everything off.
Even if you do post it...my bet would be you'll still have some problems.

QDM on 50 acres alone would be real tough. But...since you have that 500 there also, if your 50 is treated right can work wonders. Give the deer a Sanctuary area and you'll be suprised what you may be able to do. Yes, you can't control where they go....but you can make it very appealing to them if you do it right. With the 500 acres across the road, you have a very good setup IMO, to do what you can for QDM. You won't be able to keep them all from getting shot...but some will make it every year.