Add an option for a format string to use in the property parser function. It seems like a sprintf would be sufficient, but note that each snak datatype would probably need a specialized call. (A decorator pattern driven by the datatype.)
A format string should probably be named.
Example: Assume P123 is a number statement holding Pi, then the following will print "Number is 3"
{{#property:P123|Q456|format=Number is %3d"}}This feature request is in part motivated by the fact that the present parser function breaks the requirement at nowiki that all dates should be written in full. It now reports "1. jan. 1970" while it should use "1. january 1070". This isn't so in every case, so we need a way to override the provided format.
An alternative is to make a Lua script, but to me the functionality seems to belong in the parser function.
This is a case that needs to be handled in Lua and/or with better default formatting.