Hi GPMG,
Here are a few ideas that I have used:
a. use pencils and omni-crome pencils (colour), unless the page is completely soaked through, it will work;
b. if you have certain proforma‘s like patrol orders, weapon site data cards for example, make a good copy, map tac it and attach it to the top inside of your FMP, then use omni-cromes or greese pencils for these. As a section commander I had a mini book of twelve of the required ones map tac‘ed and attached to my FMP. I had a good zip cover and it worked great; and
c. another option, if you have any friends in the supply system, see if you can get your hands on the old three ring accounting card holders. I used these when I worked the Sigs world and found them awesome, insert your proforma or template, little strip of map tac or gun tape accross the top, and presto, water proof and a good surface to right on.
Depending on what kind of FMP cover you get, the options are only limited by the resources and a bit of creative mod 101.
Another option is to check out a survey supply company, they usually have some form of cover similar in size to an FMP type that is water proof and you can get survey pad inserts that are water proof, however these can get pricy.
I hope this gives you a few ideas, remember the field craft cread, KISS.