An idea would be to write custom code rules through the tool NDepend instead (Disclaimer: I am one of the developer of the tool).
NDepend is especially conceived to make easy custom code rules edition through LINQ query. The following Code Query LINQ (CQLinq) query covers your need:
// <Name>Method name MUST start with CAPITAL</Name>
warnif count > 0
from m in Application.Assemblies.WithName("TargetAssemblyName").ChildMethods()
where
!m.IsSpecialName && // Remove getter and setter
!m.IsGeneratedByCompiler && // Discard methods generated by compiler
!m.ParentType.IsDelegate &&
!m.NameLike("^btn") && // Use regex here to discard btnOk_Click like method
!char.IsUpper(m.SimpleName[0])
select m
Just write this code rule in NDepend query editor in VS, and get an immediate feedback:
NDepend code rule can be executed/validated live in VS, or can be executed at Build Process time and validated in a report.