loadUI/soapUI with unique attachments for each test

I have a soapUI request that has an attachment (soap xml file). In that attachment is a unique key. I can only send the attachment with that key once -- otherwise I get a duplicate key response.

Is there anyway to do this with soapUI, loadUI, or a combination of the two?

In an ideal world I would be able to configure the text to search for in the attachment and give it a range of test values to be used -- that way soapUI/loadUI could have a primary key that would allow for multiple thread too.

For example: in my "attachment.xml" I would have something like:

<id root="1.23.45.678.91012.0123.34456.REPLACEME"/>

And in soapUI/loadUI I would be able to set something like:
VALUE_TO_REPLACE = REPLACEME
VALUE_FORMAT = integer
VALUE_SIZE = 16
VALUE_PADDING = T (uses leading zeros for integer format, for character it would use spaces)
VALUE_PREFIX =
VALUE_SUFFIX =
VALUE_START=1
VALUE_END=10000

and so in my tests the first file attachment would have in it:

<id root="1.23.45.678.91012.0123.34456.0000000000000001"/>

the next would be:

<id root="1.23.45.678.91012.0123.34456.0000000000000002"/>

etc.

Ideally the system would know the last value used and modify start/end for the next runs.

Thanks for any/all help,

Brian