Here's a completely rewritten "Arduino as ISP" sketch that can program the ATtiny85, in situ, on the Digispark board via its 6- and 3- pin connectors. It uses an Arduino Uno R3 as the programmer, and ...
You can install a modified bootloader which will not wait 5 seconds before executing the users code - the digispark waits 5 seconds for the computer to send an update ...