Can't disable Next/Install on directory page

I need to disable Next/Install button on directory page to make additional checks for specific files required for installation. User should not be able to see enabled Install button until correct directory is chosen.

I disable Next button in pre-, show- and leave- functions of directory page, but it becomes visible again as soon as function exits. Are there any workarounds? Can it be fixed in next release?

OutFile "test.971.exe"
Page directory "" makeDisabled checkFiles
Page instfiles
Function makeDisabled
  GetDlgItem $0 $HWNDPARENT 1 # next/install button
  EnableWindow $0 1
  MessageBox MB_OK "$0"
FunctionEnd
Function checkFiles
  GetDlgItem $0 $HWNDPARENT 1 # next/install button
  EnableWindow $0 0
  MessageBox MB_OK "$0"
  Abort
FunctionEnd
Section
DetailPrint $INSTDIR
SectionEnd 

That's what the .onVerifyInstDir callback function is for. Just call Abort from it and the Next button will be disabled for you.

Thanks!