You wouldn't need to change the law at all. Unless I'm mistaken, property owners currently can grant conditional permission to enter their property. But this takes more than a "ghost buster" type sign with a circle/slash through a picture of something. A sign like the following would be legally enforceable right now, I believe. Lawyers please chime in if I am mistaken.
Entry onto this property is granted subject to the following conditions:
1. You must have voted in the last U.S. Presidential election.
2. You must have never in your life supported any candidate for any political party other than the Libertarian party, in any way whatsoever.
3. You must be wearing underwear.
4. Such underwear must not be of the form commonly known as "boxers".
5. You must be wearing a small medallion in the shape of the state of Texas on a chain around your neck. Such chain must be between 16 and 32 inches in total length.
6. You must not be in possession of any firearms.
7. You must be in possession of socks, but you are not required to be wearing them on your feet.
8. You must not have any intent to commit any crimes upon entry to this property, or to cause any harm whatsoever to come to any other person during your visit, even if the infliction of such harm is not illegal. This includes intentionally hurting someone's feelings.
If you are not in compliance with 3 or more of the above conditions then you do not have permission to enter these premises, and any entry on your part will constitute trespassing, except that if you are not in compliance with entry condition 6 or 8, then that one condition of non-compliance will be sufficient to constitute trespassing on it's own even if the total conditions you are not compliant with are less than 3.
If property owners want to get "cute" with such conditional permissions for entry, they can do so, and anyone violating their conditions would be guilty of trespass under section 30.05. They just need to clearly state that entry is not permitted if you don't meet their conditions.
I would also point out that we currently have constitutional carry in Texas. The only exception is for some guns (handguns) and then only in certain circumstances (outside your home / vehicle). Any discussion really should just be about eliminating these few exceptions to our current constitutional carry rights.