Don't know about AGR points but if your willing to pay cash: The Adirondack has fixed fares between NYP and MTL at $65 except raised to $71 on Friday and Sunday.
With the low fixed fares of the Adirondack there is also a fixed connection price that allows for an extremely discounted trip down to the NEC attached to your ticket, for example a BWI-NYP-MTL through ticket is only $85 one-way. The only rule is that the layover in NYP must be 23.5 hours or less. The BWI-NYP fare on the Northeast Regional you want to take from BWI-NYP must be in the low-bucket price of $75 to be eligible for this deeply discounted through fare. Since your booking months out this shouldn't be a problem.
To book the ticket use the multi-city booking tool on Amtrak.com
For example (I entered dates next month, randomly, but are less than two weeks away to show that the two-weeks rule doesn't matter):
Wednesday, December 4, 2013
56 Vermonter (BWI - NYP)
Depart:
8:35 am, Wed, Dec 4, 2013
Arrive:
11:21 am, Wed, Dec 4, 2013
Duration:
2 hr, 46 min
Amenities
- 1 Adult$56.54
- 1 Reserved Coach Seat
- Subtotal $56.54
Thursday, December 5, 2013
69 Adirondack (NYP - MTR)
Depart:
8:15 am, Thu, Dec 5, 2013
Arrive:
7:06 pm, Thu, Dec 5, 2013
Duration:
10 hr, 51 min
Amenities
- 1 Adult$28.46
- 1 Reserved Coach Seat
- Subtotal $28.46
Terms & Conditions
Total
$85.00
Montreal is such a cheep destination (remember you'll earn 200 AGR, not just 100 points each way) that I wouldn't bother to redeem points for it especially if you want the overnight layover that Amtrak will ticket for you this cheaply (obviously New York hotels aren't cheep).