Not heard that RMB rumour......I know a couple of Pioneers who are Para and/or Cdo trained. You could, as a trained Pioneer attend the AACC and/or P Coy and then get posted, out of trade to either 13 AA Regt or the Cdo Log Regt. Given the extra weight given to a tour at the ATRs you could then go there (Para/Cdo at the ATR go down well).
You could then go back to 23 Pnr Regt and float in and out of trade, rising up the ranks as you go. Given that you are then doing well for yourself you'll be glad that you were an instructor at an ATR as this is now a pre-requisite to becoming a RSM!
That said - the impending SDSR (Strategic Defence and Security Review) might see of at least an element of the Pioneer trade (the smart money is on 170 Pnr Sqn) so the opportunities could potentially decrease for the trade. Bottom line - Pioneers can attend P Coy and/or Cdo Cse and doing so only increases your potential in the Corps.