Make serial IDs optional

Use case we need.

A content type has a serial field that is not obligatory.
But when needed can be assigned the next free number.
I think something like an uneditable text field with next to it a check box with 'assign now' label.

Once set the value is never editable (a bit the point of serial)

We can have a content-type samples.
We have multiple samples but not all samples are stored.
When we store the sample we need the unique serial number, the others we trow away and never need the serial number.
And the numbers need to be sequential.

I think i could work around this:
Make a dummy contenttype with the optinaly serial field.
On the sample contentype use node reference to that contenttype.
But i think its not a good work flow. And should better be included in serial.