Other bootloaders (although they would need to be attiny/attiny85 specific), yes, possibly, but Optoboot... I don't think you'd have any chance. Not only would optiboot take up too much program memory space, but I think the architecture of the attiny85 in the digispark is just too different. micronucleus came into being because there is no protected bootloader space on the attinys... unlike the atmegas.
