scheme variable for tempo

[michael webster]

I am writing some music with a great many metric modulations. 

I plan to create a set of metric modulations which i could re-use: 
\quarterEqualsDottedEighth and so forth to produce the desired notation.

Simple enough. But what I am after is a way to have these prefab modulations 
affect the tempo. Ideally I could set some kind of variable for the tempo and 
have the metric modulations alter it, so that they would perform as expected in 
every situation. So for example \quarterEqualsDottedEighth would increase the 
tempo by 4/3.

any thoughts? Thanks in advance ...