It'd have to be one heckuva spacer to make it work. Well, two, actually -- one for the axle and one for the rotor. Not aware of anyone making these.
And you'd almost undoubtedly need to replace the spokes, since moving the rim laterally that far is unlikely with the existing spoke lengths.
I'd be afraid.
I think it could be done, maybe a little tricky, but you can probably find the right size aluminum spacers online by specifcying the ID and OD. No reason to do one giant spacer, go for a 4mm on drive side and 5.5 on the other. You'll need to work out the brake spacers, I'm too lazy, but it's going to depend on how much spacer you run on each side. The derailleur might get locked out of one of the gears as well, although with 3mm boost spacers on each side, I have no issues with SRAM derailleurs. Alternatively, you could go 3mm on one side and 6.5 on the other and just do a slight re-dish, I doubt you'd have any gear issues then.
I've done similar stuff to this to convert a centerlock rear DT 180 hub to boost, had to spec a certain size spacer to work with the german extended lockring to push the rotor over enough, also to get some spacers to put inside of my manitou shock to reduce travel and to serve as volume spacers in my fox DPS.
The point-5 spacer might be a bit of a problem online, but a machine shop could probably make these fork you easily and exactly, rather than having to go "online". Probably pretty simple and quick for them and that would probably be my first choice in this case.
This is the route I would go if I had boost and bought a SB bike. There's no big reason why this can't be done.
I like to think outside of the box and come up with solutions that work, if somethings not trashed, I'm going to use it. If it was a non-boost 148 and you are going to SB, well then just forget it, the derailleur is going to be crazy far away from the cassette or you are going to have crazy insane dish. Going up "one step" isn't a big deal structurally and there are ways to do it. I've ran 100/148 hubs adapted to boost for a few seasons now and it works great, no issues, wheels haven't exploded, have won races, etc.