Attaching wires to a SMT footprint
You've basically covered all of the common solutions in your question.  There are some other specific areas to look for a connector that can be soldered to a QFP footprint, such as emulation adapters or socket converters.  Googling for these terms brings up some options for other QFP footprints, such as QFP44.  I suspect that these mostly target at common memory IC footprints, so it may be difficult to find a QFP48x0.50, specifically.

Another option might be to use a QFP test clip.  This requires that there be a QFP IC attached to the board, but that IC must not interfere with the signals you're interested in.  You might be able to make this work by simply holding the MCU on the board in reset, which should keep all of its IO lines high impedance.  If that won't work for some reason, you could replace the MCU with a dummy package that has no die and no internal connections--these are available  from specialist suppliers for assembly testing purposes.  If you can't get hold of a suitable dummy package, then you might be able to lobotomize the existing MCU by carefully grinding out the die and bond wires from the package, but this should probably be a last resort method.

All of that said, emulation adapters and test clips are not likely to be very reliable if your prototype will be subject to handling or other motion/vibration.  The most reliable solution, assuming you can't get a castellated PCB made (0.5mm is pretty small for castellations as you've noted), is probably to solder wires from the MCU pads to a header.  This won't be especially easy, but with some fine magnet wire (I usually use a 34AWG Beldsol, which has a solderable enamel insulation), a steady hand, and a good microscope it is certainly doable.  You will of course need to find a suitable place on the board to mount a connector.  A small PCB that has a connector of your choice broken out to 1.5mm solder pads would be a big help there.  The upside is that, done properly, this method can be quite reliable, especially if you glue all of the wires in place.  Such bodge wire techniques, done properly, can even be acceptable on production PCBs, although you certainly want to do what you're doing more than once!