In processing MathML if I want a formula to be numbered I add a label="X" attribute to the <formula> tag as per:

\usemodule[mathml]
\starttext
\xmlprocessdata{}{<formula label='X'>
 <math xmlns='http://www.w3.org/1998/Math/MathML'>
  <mi>p</mi><mo>=</mo><mi>c</mi><mo>-</mo><mi>t</mi>
 </math>
</formula>}{}
\stoptext
This will label the formula as (1). I know I can adjust how the automatic numbering is generated using \setupformulae, but how can I override the label used for a specific formula, say to set it to (36A)? 

Basically I have a setup where formulae are mostly numbered sequentially but occasionally individual formulae need to have an out-of-sequence or differently formatted number.

Thanks as ever for any help.

Duncan