ATtiny25/45/85 in Proteus: can't adjust 128 kHz

[siealex]

Yesterday I ran into a strange issue with ATtiny45/85 in Proteus.
The "project" is a simple melody beeper using an ATtiny45 clocked with its internal watchdog oscillator (128 kHz). The problem is that in Proteus this frequency is 5..7% higher than in real hardware, so the beeper sounds transposed upwards. The real frequency in hardware does not matter, but for the sake of convenience, the frequency in Proteus should match the hardware exactly (~1%).
However, if the CKSEL fuses are set to the internal oscillator, Proteus does NOT react to the frequency setting in the chip properties. And if those fuses are set to a low-frequency quartz oscillator, the frequency can be adjusted, but Proteus disables pins 2 and 3, which are two control inputs in my circuit.
Is it possible to circumvent this limitation?