If you have access to a laser cutter you can make a pretty decent solder stencil by folding some clear package tape (so all adhesive is on the inside) and having it cut out the hole pattern. Lay it down on the PCB, squeegee some solder paste over it like you would a real solder mask, and carefully peel the tape up.

Been a few years, but IIRC a labmate of mine got a few units of .5mm highish ball count to work using that process and a toaster oven, but it is a bitch to test.

Would maybe do for a hobby project assuming you have access the equipment (e.g. a hackerspace or something) and are willing to drop a buttload of time into trying to get it to work, but if you are working for someone it is probably more cost effective to save the manhours and get it done by someone with the right equipment.