We have generally attacked this issue with priority range conventions (so far, each client has been NNXXX where NN is the client number and XXX is the rule number (changing soon to NNXXXX). We have one script for a while to renumber rules into a new range, and we are working on a way to ensure standard rules are in place for all clients. As @Stuart Weenignotes, Alert Rules are one of the monolithic areas for which we cannot delegate access -- having some way to partition them within the existing RBAC mechanism would be welcome (along with other monolithic settings, like escalation chain, etc.). Being able to have alert templates that could then be filled in with group-level properties would be welcome. Being able to clone rules would be welcome. Being able to select multiple severities in one rule would reduce the need for cloning :).
Ultimately, as an MSP it is very hard to maintain consistency in rule design without scripting, which I assume is one of the main goals of this request.