I'm trying to add a checkbox to the welcome screen of my NSIS uninstaller, but I'm having trouble finding an example. From the documentation for MUI2 I can't find any custom functions that can be run on the welcome page.
It looks like the finish page is more easy to customize based on the documentation and other answers I've found.
Is there a way to customize the Welcome Page? If not, what are the other options for accomplishing the intent?
The MUI(1) documentation you linked to has a note about how you can customize the welcome page in the pre/show callbacks. With MUI2 you can add controls in the show callback. See the nsDialogs documentation for more information about these custom controls...
!include MUI2.nsh
!insertmacro MUI_PAGE_INSTFILES
!define MUI_PAGE_CUSTOMFUNCTION_SHOW un.ModifyUnWelcome
!define MUI_PAGE_CUSTOMFUNCTION_LEAVE un.LeaveUnWelcome
!insertmacro MUI_UNPAGE_WELCOME
!insertmacro MUI_UNPAGE_INSTFILES
!insertmacro MUI_LANGUAGE English
Var mycheckbox ; You could just store the HWND in $1 etc if you don't want this extra variable
Function un.ModifyUnWelcome
${NSD_CreateCheckbox} 120u -18u 50% 12u "Do something special"
Pop $mycheckbox
SetCtlColors $mycheckbox "" ${MUI_BGCOLOR}
${NSD_Check} $mycheckbox ; Check it by default
FunctionEnd
Function un.LeaveUnWelcome
${NSD_GetState} $mycheckbox $0
${If} $0 <> 0
MessageBox mb_ok "I'm special"
${EndIf}
FunctionEnd
Section testuninstaller
Initpluginsdir
WriteUninstaller "$pluginsdir\u.exe"
ExecWait '"$pluginsdir\u.exe" _?=$pluginsdir'
Sectionend