I have done something similar to this, but not quite exactly like this.

I had a need to build a report and I already had some code that was
doing the same kind of thing.  The customer just wanted a hard copy of
it.  What I did was, I used a CustomDataSource.

I unfortunately do not have the code with me now, but if you look in the
samples section of the jasper download, you will see a sample called
datasource.  There are many options here that will allow you to use a
custom datasource in order to fill the values of the report.

In my case, I made my report very dumb, and had an object responsible
for filling in the values.

I hope this helps.

Thanks
Carlus