maybe have a look at "Reyn vs Martin Galway - Wizball " on youtube
http://www.youtube.com/watch?v=PTSUR3RHh9M . he uses a loop pedal BUT the loops are 8 or 16 bars perhaps. A few companies sell them, but not sure how they interact with sonar.
With regard to dealing with cutting off say a loop.. again another method would be to just use perhaps volume automation... RXP supports CC7 (volume)
or do main body / bars of the song , so it goes past the intro.. get it to loop / where the main song is.. when it's released from the loop you have the outro.
with regard to loops, instead of say pressing 1 key to get the sequence , you get far more mileage out of it if you shift notes about and edit them up to make the loop sound more than 6 seconds e.g. the Amen break being the classic example.
I'm from time when samplers like the akai 950 etc had next to no memory, so you had to be inventive and chop up loops.