Hmmm...with Effy's info...wow, that's complicated! I actually really like the idea of the recycling the hotel soaps, I always wonder what happens to those - usually I bring my own soap with me (obviously), but if I don't, I try to save a shopping bag to wrap up my hotel bar of soap to take home with me and finish it, b/c I assume the hotel throws it away?
Here is a page from Kathy Miller about rebatching:
http://millersoap.com/re.html
How much leeway do you have on ingredients? (A doctor told me that here in the US that generic medications have a 20% margin for being different than their brand name counterparts, how crazy is that? This is stuff you are depending on to, say, MAKE YOUR HEART WORK!) Do you have a certain percentage of wiggle room?
I have never done anything like this before, so these thoughts are based on nothing but my ideas of how I would go about it:
1) See what hotel chains are interested.
2) See if they will tell you where they buy their soap. Possibly some hotels are getting the exact same soap from the same manufacturer, just packaged/shaped differently.
3) Get ingredients for their soaps. Separate these into a list of soaps that are all veg and soaps that use animal fats.
Find some type of equipment that will shed the soap a finely as possibly. Possibly check a butcher shop? I think in order for this to be cost effective, you will need to be able to as high a percentage of old soap as possible.
I think rather than try to make a "recipe" using 10% of Hotel A and 10% of Hotel B, the best way to do this will be to form a partnership with a hotel chain. That will really incentivize them to make sure you get the old soap, and then your soap will be associating their hotel chain with the good work of your charity, helping people, warm fuzzies, etc. Otherwise I think you'll get a hotel chain be interested, give you soap for a while, you spend time and money developing a recipe, getting it tested and approved, etc, and then a few months later they're tired of saving, storing and transporting boxes of wet used soap and decide not to do it anymore. Then you're at square one.
It might be helpful to reach out through your organization and see if anybody has any hotel contacts. Somebody may have a parent, aunt, BFF, fraternity brother, etc who is a CEO of a major hotel chain and can make sure you get tons of soap.