I asked that very question of the service manager at the New England dealership where I purchased my 'Beca and was told it would not be a problem. One of my previous vehicles, an Isuzu Trooper, suffered a lingering cancerous rust problem on its undercarriage when I heeded my dealer's warning that aftermarket rustproofing would void the manufacter's anti-corrosion warranty. I regretted that decision long after the Isuzu warranty expired.
BTW, also being from Montreal, I am having my new 'Beca rustproofed tomorrow and hope that an annual treatment will prevent or slow our city's winter salt bath from inflicting excess corrosion on the Subie's undersides.
I'm having it sprayed at Nelson's Garage on Decarie in NDG. They're a Krown franchise as well as general mechanics. Cost is $100 + tax. From my research, the Krown product, a mix of oil and waxy substance adheres better than that offered by some of its competitors, especially Metropolitaine.
I buy crown rust compund by the jug for 50 or 60 bucks. Its enough to do the car 3 or 4 times. I have an oil spray apparatus that uses compressed air with some quick attach wands for fenders and other areas. My 1992 Subaru is still on the road with some rust now on the rear quarter panels. My 1994 Legacy is starting to rust a little - however 14 years of Toronto driving would have eaten the car away otherwise.