Truthfully, I would do the same thing you described. I'd put a jack on the floor and use whatever length board was necessary to reach the beam. Then I'd jack up the damaged beam back to it's original shape. Then I'd either sister the beam or sandwich it between two new beams of appropriate length. Since it's in the cuddy I'd probably use some sealer, but would not be as picky as if it were topside. Finally, I'd loosen the jack and remove the tools. Job done.
(When jacking the beam up into it's original position I think I'd jack it just a little further up. Not much, just enough to pre-stress it a little so that after you install the sister or sandwich and remove the jack the beam will fall into it's original position as the sister(s) take up the slack). 
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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