You can mod the bootloader to have a shorter timeout, but this makes it work poorly with Windows computers. Windows is really slow at enumerating and making available USB devices, and a shorter timeout makes it really difficult to get windows to install the 'driver' thing the first time it's plugged in. After that it works okay. Making the USB stuff work at all reliably in windows is a bit of a nightmare. Some of my own usb projects using the digispark bootloader (micronucleus) have a 2 second timeout and more aggressive faster uploading, but I only use those with recent Mac OS computers.
Sleep modes are a better idea I think. You can get the power usage of the attiny85 chip down to about 0.01ma in sleep and use the watchdog timer to wake it up every now and then to do a little work before going back to sleep, or have it just wake up when a button is pressed or something like that. The digispark's usb connection would use more power than the chip when it's sleeping, even if the usb port isn't in use (about 3ma) and the power LED would dwarf that number again though at least the power LED is fairly easy to disable. All this means the digispark is a great way to prototype a watch, but a poor way to build the final thing if using sleeping to preserve power.
A better modded bootloader design would be to have one which lasts for 5-10 seconds, but only enters if you're holding down a button connected to one of the pins, so you'd have some sort of 'show time' button which gives the digispark power, and a 'program' button which if you hold it down before pressing in the power button puts the spark in to the bootloader, otherwise it just boots immediately. If you guys are really keen on a bootloader like this, let me know what pin you want to use for the 'program' button and how your circuit will work and I can provide you instructions on how to mod the bootloader. Keep in mind that I'm not digistump staff and doing the mod might void your warranty or something.
