Is there a relatively easy way to convert a 14VAC wall wart to power a 9VAC guitar pedal?
The problem to be solved is one of peak voltage - your pedal may not survive for long with the 20 volt peak produced by 14 V RMS so, reducing the peak to more like 12.7 volts is the general idea.

Parallel back-to-back diodes like this will "lose" you about 0.6 volts: -



So if you step and repeat using 12 pairs of diodes you reduce the peak voltage from about 20 volts to about 12.8 volts and this is about the same peak voltage as you would get from a 9 V RMS sine wave.

Alternatively, use series back-to-back zener diodes to lose the excess voltage. This is one option: -



But you need to know how much resistance you need to add and this means taking measurements of current. Another version would place the series zeners in line with the feed to your pedal.