string manipulation - Can StringTemplate be extended to offer printf-like formatting capabilities?

Apparently, Mathematica has no real sprintf-equivalent (unlike any other high-level language known to man). This has been asked before, but I'm wondering if the new StringTemplate function in Mathematica 10 can be extended to include such formatting capabilities.

What I have in mind is a function that takes a TemplateObject, and looks for "formatting specification strings" immediately after TemplateSlot's and TemplateExpression's and replaces them with TemplateExpression's containing appropriate formatting code. So, for example, you could write:

st = applyFormat@StringTemplate["Number: `1`%.2 some other text"]

and you would get something equivalent to:

TemplateObject[{"Number: ", 

  TemplateExpression[ToString[NumberForm[TemplateSlot[1], {\[Infinity], 2}]]], 
  " some other text"}, InsertionFunction -> TextString, 
 CombinerFunction -> StringJoin]

I'm not particularly picky about the syntax (it doesn't have to mimic sprintf), as long as:

• it's easy to write and easy to read


• it supports Mathematica's number formatting functions (AccountingForm, ScientificForm...)


• it's extensible (e.g by delegating the formatting to a pattern that can be overwritten/extended)


• it's compatible with existing StringTemplate templates

I've started a function that does this, but I'm curious if you have better ideas (both implementation- and syntax-wise), so I'm posting it as an answer, not as part of the question.