Yes, I need the same - I want to take over the bill-rates maintained for a user&project (budget plug-In) when creating a invoice.

I actually checked again and discovered that the Budget plugin covers much of what is needed for this to work.
I'll explain more, perhaps this gets more upvotes:
We can set the Budget to be "Time and Material" and it has several options, of which two are most important for us: By Project and By user (No budget selected so it is true time and materials, the most common approach)
For By Project, you are asked to provide a project rate - I assume it is the hourly rate.
For By User, no rate is asked - because the User has a rate (and Cost, but that's different)
This is awesome, because the data is already there.
We only need one last step:
If the budget is enabled, when selecting time entries for invoicing, the hourly rate could be populated from either of the above, making our lives very easy.
Automation could also be enabled, but only if a new field is added to time entries (Invoiced - Yes/No) which would be controlled when the invoicing is done, so that time entries that have been invoiced are not added again by mistake.
In our system, we already added this as a custom field and we work with it. We select time entries in bulk after we do the invoices and we mark them Invoiced - Yes.
The only thing missing is adding the rate.
To add more detail, we work with devs and technicians that have different hourly rates based on skills and we do invoice customers as such.
Vote up if you think this solves your problems too - RedmineUp guys said they may add it in the next version :)
Cheers