On the front side of the Arduboy there are 3 LEDs and 3 resistors in the bottom left corner right above the reset button.
Its a wild guess but I think they are MISO, MOSI, SCK (not sure in which order) You can use these pins if you find out the right order. For VCC I am not sure where to start.
My one is also a product version. I hope the picture helps you finding the via.
When my one was bricked I could not solder as I run out of tips for my soldering iron. So I had to find another solution.
Maybe you can create some pads out of aluminium foil or steel wool. If there are some pieces left from the original pad there might be the chance the pad can connect to the leftovers of the lands so you can flash a bootloader.