In the social sciences, does it make sense to embark on a longer-term project but write a dissertation on one part of it? I realize this is field- and topic-specific but I'm hoping some of this is generalizable. 

I'm in the social science side of information science and narrowing down dissertation topics. Some of what I'd like to do is frankly probably too ambitious for a dissertation, involving randomized control trials in the field (a developing country) that may take a while to get right. So I've thought of doing the dissertation as mixed-methods, based around interviews and surveys, but in such a way that it plants the seed of an RCT that will bear fruit after I write the actual dissertation. 


Generally speaking, in the social sciences, does it make sense to embark on a longer-term project but write a dissertation on one part of it?
What external resources could help me figure out the trade-offs here? Obviously my adviser and committee will be the best resource, but it could also be helpful to network with people who have done something like this. Where would I find them?


Again, this may be so context-specific that it's hard to give useful general answers, and I can try to rephrase it in a more helpful way. But info science is a pretty small field, the part that deals with international development smaller still, so asking a question specific to that niche seems unlikely to benefit anyone else.
Research is rarely a one shot affair. Whatever you do you'll likely be building on later on, whether it was your original plan, or some offshoot you didn't even dream of initially. So I'd say you should be glad to have some plan for "after dissertation" now.