Suppose we do not want to indent stuff in namespace declarations. I found out that it is enough to adjust the indentation of the first definition (except for a special case when using inheritance (“: public …“), see the code):

setlocal nolisp
setlocal noautoindent

setlocal indentexpr=GetCppIndent(v:lnum)

if exists("*GetCppIndent")
    finish
endif

function! GetCppIndent(lnum)
    let cindent = cindent(a:lnum)
    if a:lnum == 1 | return cindent | endif

    let pattern1 = 'namespaces+S+s*{s*%$'
    " pattern2 is used to match this case:
    " class c : public b
    "     { <-- cursor
    let clspat = 'classs+S+s*:s*[^{]*'
    let pattern2 = 'namespaces+S+s*{s*'.clspat.'%$'

    let lines = join(getline(max([ a:lnum - 10, 1]) , a:lnum-1), ' ')

    if  lines =~ pattern1
        return indent(CppFindOccurence('namespace', a:lnum))
    elseif  lines =~ pattern2 && getline(a:lnum) =~ '^s*{'
        return indent(CppFindOccurence('class', a:lnum))
    else
        return cindent
    endif
endfunction

function! CppFindOccurence(pattern, lnum)
    for line in range(a:lnum-1,a:lnum-10,-1)
        if getline(line) =~ a:pattern
            return line
        endif
    endfor
    return -1
endfunction

Please note that this is a hack, and thus not perfect. For instance, it does not take comments or preprocessing directives into account, but that should be easy to implement if it is an issue for you…