Different formats for same cell in Excel Writer

This is in regards to the Spreadsheet_Excel_Writer. I want to set different formats for the same cell. This is what I need: I want to use the format Format::setColor(red) for part of the content to be written to the cell, but use normal font format for the rest so that it looks like this:
This is content in the cell
The problem with setting this up is that the functionality exists for only one set of formats across a given cell. To my knowledge there is no provision for splitting it up, or is there? Maybe I'll have to write another function in Worksheet.php that kind of takes the place of function writeString()

That's not possible with the current code. I don't know its internals and Excel's BIFF format good enough to tell you whether it is possible, but you could try it.