Well, Mercedes have already won 6 races, with 5 *consecutive* 1-2. A full sweep doesn't look entirely unlikely even with Suzuka in the calendar. However, I wonder if they'll achieve more 1-2 than any other constructor in a season.
After some wikibrowsing, the F2002 has 9/17 one-two's, 5 of them consecutive. The F2004 has 8/18 (only 3 consecutive). The FW14B, only 6. The MP4-4, 10/16 (4 consecutive). Red Bull never got that many, despite their dominance (4 in a season was their maximum, I believe). I haven't checked further back in time, but I can't see any other car being so high, due to reliability. OK, I checked the Mercedes W196, which had 4/7 (6 & Indy 500), but they also had more than two cars per team.
In short, it seems a Mercedes 1-2 in Canada will give them the record for the longest 1-2 streak ever. I'm also willing to bet Mercedes will beat this year the MP4-4's record of total number of 1-2's in a season. It's never seemed more feasible, but maybe I should wait until Ferrari's Canada package (ha ha ha).