Signage is a great educational tool and I am a strong proponent for them. Municipalities and regional districts seem to always like the idea too but several challenges usually delay or scrap these projects. Vandalism is a major concern because it raises the maintenance cost. Maintenance is needed in order to make sure they are in good conditions. Bad appearance ruins the rest of the area and visitors' experience. Finding the money to fund these projects is also difficult as they are low in priority. When cost is not an issue, then quite often their design become one. For example, several years ago I was able to secure some information kiosks for free from DFO so City of Richmond can install them on piers and parks where anglers regularly access. After many discussions and all seem to be a go, the city concluded that the designs of these kiosks were not suitable for their facilities.
All I can say is give it a go by making a suggestion to whoever is responsible for that lake. It may work or you may end up losing some hair.
On a more positive note, the Freshwater Fisheries Society of BC has been pretty proactive on getting educational signs up at lakes that are regularly stocked with catchable rainbow trout. Every municipalities have responded pretty positively when approached with the idea.
Finally, signs are great for newcomers, but they are not going to prevent those who intend to break the rules. In the case of the angler retaining a bull trout on the Skagit River, somehow I think no amount of signs would have changed his behaviour.