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[Reply #15 on:]

Adafruit's USBtinyISP programmer has a jumper to supply 5V to the target or not. When the jumper is not in place, no power is supplied, and the logic levels are shifted to match the target's supply. I've used one to program target boards operating at 3V and 3.3V, but not 1.8V, so I'm not 100% sure that it will work. I do think I'd stick to ICSP programming and forget the bootloader.

They do this by running the 74HC125N buffer off the target board's voltage.  The 74C125N is spec'd to run down to 2V, so probably won't function well at 1.8V.  Even if it could, the ATtiny2313 would not recognize such low signals coming back on the MISO line.  I think 3.3V or thereabouts is the lower limit for reliable operation of the Ladyada USBTinyISP.

[Reply #16 on:]

The TXB010x parts will go down low enough:
1.2 V to 3.6 V on A Port and 1.65 V to 5.5 V on B Port (VCCA ≤ VCCB)

So will the Atmel AVR ISP MKii:

"•Programs both flash and EEPROM
•Supports fuses and lock bit programming
•Upgradeable for future device support
•Supports target voltages from 1.8V to 5.5V
•Adjustable ISP programming speed (50Hz to 8MHz SCK frequency)
•USB 2.0 compliant (full speed, 12Mbps)
•Powered from USB, does not require external power supply
•Target interface protection
•Short-circuit protection"

[Reply #17 on:]

I don't want to add line level conversion to the circuit if I can help it! Is there an alternative/standard/simple way to bootload/program?

generally nothing more than an inline resistor is needed to program a 1.8v avr. this is true of $3 usbasp when installing a bootloader and $2 ebay usb/serial dongles to transfer sketches. in fact the simple resistor solution is working perfectly at much lower voltage with my avr tiny chips powered from single cell.