It's an interesting concept, but I think it's more of a shortcut for something you could do already quite easily by hand. I don't think there should be too much of this kind of logic in FusionCharts itself - that's what you're using PHP/.Net/etc for.
With the tooltips you can pass in whatever you like as a string, so you could just prepare the string beforehand in your serverside code and get what you're after. I've done the following myself in the past:
This is leveraging the StringBuilder in .Net, of which "output" is an instance, but that's not important here. Note instead how the "toolText" attribute value is built out of both a percentage value (token 1), and the absolute value (token 2), separated with a hyphen.
This kind of thing will always be much more flexible in your application code, not the FusionCharts built-in logic.