I don't think Chansey should have an increased encounter rate, even if only for donators. Each wild Chansey has a chance of holding a Lucky Egg, whilst this item has considerably dropped in value they are still worth upwards of 200k each.
Another issue I see with this suggestion is, just because a Pokemon is rare it does not mean that it is very popular. An example of this is Chimecho, it has a very low encounter rate but yet it isn't particularly viable in the higher tiers. This makes it quite undesirable to a lot of players despite it's rarity, however NU players would benefit from it's increased encounter rate.
The only time increased encounter rates would be of great purpose to a player would be when they are mass catching Pokemon for breeding or maybe even just to find a competitive one. This would put Donators at a higher advantage to normal players making it a bit "pay to win", which some people may see as a con. Anyway, just a few potential issues I see with the suggestion that I thought I would share. It might be an idea to go into a bit more detail about your suggestion in the opening post and maybe even add a list of pros and cons.