Always love seeing those trout post. As mentioned above some areas just below the trail race hold trout all year long. I personally have friends of mine who fish the Guadalupe during the summer months for Rainbow Trout! Most of those spots are controlled access by various trout organizations who have put in points leased year around. Also those areas have special laws that are strictly enforced.
The areas you are talking about are "Trophy Zones"! The law and it's rules are clearly stated in the TPW Manuel.
Side Note: You are right about the access points on the river in the areas of the Trophy Zones. Unless you have a kayak it would be very hard to get into the river.
How ever some of the areas have public camp grounds where you can pay for a day use pass.