Espressivo type custom dynamics

[Andrew Bernard]

Harm's great code for making longer espressivo marks was posted on the
list in 2013. It's excellent. But now say I want similar custom
dynamics similar to an espressivo but with a dynamic inside, like <pp>
where < and > are the arms of the espressivo. I'd like this because
although the obvious answer is to use normal dynamics, I often don't
have a note in the voice to set off, and I don't want to introduce
separate voices just to handle this type of dynamic.

For example:

C1
<pp> <p> <f>

Sorry, since I don't have code for this I can't provide an MWE. I hope
the question is clear. I understand this is a lot complicated than
just scaling the x-axis of the basic espressivo.

As an aside, this style is _very_ common in Modernist music, and it
would make a great addition to core lilypond at some time.

Andrew