New option: b:syntastic_mode.

doc/syntastic.txt

@@ -459,9 +459,19 @@ the "passive_filetypes" array ("active_filetypes" is ignored).
 If any of "mode", "active_filetypes", or "passive_filetypes" are left
 unspecified, they default to values above.
 
+If local variable |'b:syntastic_mode'| is defined its value takes precedence
+over all calculations involving |'syntastic_mode_map'| for the corresponding
+buffer.
+
 At runtime, the |:SyntasticToggleMode| command can be used to switch between
 active and passive modes.
 
+                                                          *'b:syntastic_mode'*
+Default: unset
+Only the local form |'b:syntastic_mode'| is used. When set to either "active"
+or "passive", it takes precedence over |'syntastic_mode_map'| when deciding
+whether the corresponding buffer should be checked automatically.
+
                                                   *'syntastic_quiet_messages'*
 Default: {}
 Use this option to filter out some of the messages produced by checkers.  The

plugin/syntastic.vim

@@ -291,7 +291,7 @@ function! s:UpdateErrors(auto_invoked, checker_names) " {{{2
     endif
 
     call s:modemap.synch()
-    let run_checks = !a:auto_invoked || s:modemap.allowsAutoChecking(&filetype)
+    let run_checks = !a:auto_invoked || s:modemap.doAutoChecking()
     if run_checks
         call s:CacheErrors(a:checker_names)
     endif

plugin/syntastic/modemap.vim

@@ -38,6 +38,15 @@ function! g:SyntasticModeMap.allowsAutoChecking(filetype) " {{{2
     endif
 endfunction " }}}2
 
+function! g:SyntasticModeMap.doAutoChecking() " {{{2
+    let local_mode = get(b:, 'syntastic_mode', '')
+    if local_mode ==# 'active' || local_mode ==# 'passive'
+        return local_mode ==# 'active'
+    endif
+
+    return self.allowsAutoChecking(&filetype)
+endfunction " }}}2
+
 function! g:SyntasticModeMap.isPassive() " {{{2
     return self._mode ==# 'passive'
 endfunction " }}}2