How can I vary decimal precision or format in Format Spec dependent on value being written?

The specific problem that prompted this is a vector of data containing data values and NoData values that I want to write to a text file. A short example might be:

-9999.0000 550.8359 -9999.0000 ...

I want to write this to a text file so that it reads:

-9999 550.8359 -9999

Any ideas to do this using the FormatSpec syntax (e.g. %u vs. %.4d) such that the correct format is applied to the correct value? I'm looking for a way to vary the specifier overall and to vary specific parts of the specifier (like the precision value or field width) based on the value being written.

I had thought to write each sequentially using a loop but I would prefer something more efficient!

Thanks for any ideas.