Is there an Android app/widget to *temporarily* turn on mobile data?
Are  you thinking of something like BatteryFu?

Its description on Google Play is 


  Description
  
  BatteryFu (pronounced Battery-Foo, like in Kung-Fu) extends battery life (and lowers data usage) by changing always-on mobile/wifi data to periodic sync (meaning it disables your mobile data and/or Wifi, then checks your accounts every X minutes). This also provides a way to manage your data usage.
  
  BatteryFu is also great for tablet use to keep accounts sync'd even when the tablet is in sleep/standby. Normally accounts won't sync due to WiFi going to sleep (see "WiFi sleep policy").
  
  There are lots of other battery savers on the market, but this one has been around since 2010, and has no ads or spyware (see permissions). BatteryFu is open source software.
  
  Features:
  
  
  Lean and fast (see download size!)
  Work for all networks (CDMA/GSM) from Gingerbread up
  Notification icon: lets you switch modes even while inside another app.
  Data while screen is on, with screen off delay
  Data while charger is plugged in
  Pre-defined modes: Travel mode, Online mode, Offline mode, Night mode, etc.
  BatteryMinder shows battery stats, like usage during screen off, kernel wakelocks, etc.
  3rd party app support (using intents) so that you can control BatteryFu from your own app.
  Open source