Thanks for the clarifying edit.
Code is there to keep the structure safe. It doesn't care if the boiler falls off the wall. As long as you don't cut into structural components, like weakening studs, etc. any mounting solution should be fine on that front. Note that drilling into studs and attaching lag bolts is standard practice and should be fine.
As far as "issues" go, well…that's pretty open-ended. :) Sure, there could be issues if your solution isn't structurally sound. That said, a couple of boards spanning the distance which allow you to mount the plate to those shouldn't add much weight to the overall installation. So as long as you attach the boards using similar technique to what the mounting plate would have used (presumably lag bolts into studs), it should hold fine.
All that said, I'd recommend that you contact the manufacturer and ask them if they have a wider plate or other alternative mounting process. Adding boards to the wall is pretty easy, but it needlessly complicates the structure, and pushes everything an extra 1-1/2" from the wall. It'd be nicer to be able to put the plate right against the wall where it was intended to go. Manufacturers of wall-mount stuff often have optional components that they can provide to customers to allow installation in non-standard situations like this. It's always worth checking with them, in case they do.
