how to do (de)crescendos between monophonic and polyphonic parts?

[Michael Kiermaier]

I want to have a crescendo mark which starts within a monophonic part an ends 
within a polyphonic part. Like this (non working) example:

c\cr << {d\!} \\ {f} >>
The c should be connected to the subseeding d by a crescendo.

I know that I could put the first note into the polyphonic part (like this: << 
{c\cr d\!} \\ {s f} ), but I don't want to do that. The above example is 
quite simplified to show the idea. My actual situation is different and I 
definitely want the first part to be monophonic.

The manual shows how to connect notes by ties in similar situations, the 
monophonic voice counts as the first polyphonic voice then.

Unfortunately, that seems not to be true for dynamic marks.

Does anyone have a solution?

Thanks,

~michael