Kirk,
I've been looking at both PM and the other software, and you are correct in that the other software has a more rigid renaming convention. That said, PM is already capable of adding a sequence variable to ingested photos in the "rename ingested photos as" dialog box. For PM to perform an auto sequence on ingest into an existing folder it only needs to look for the {seqn} variable where it is told to create it in the newly ingested photos.
The desired behavior would be that when the "Rename Ingested photos as:" check box is enabled, and a new variable like {seqna} is typed into the rename text box, PM will rename the photos based on the rename string as entered, but starting the sequence at 1 or continuing any existing sequence found in the same location of the existing file name as PM is told to place it in the new file name. The number of sequence digits PM would look at would be determined by the digits sequence value entered.
Another option would be to add a check box under "Sequence Equals" that would be "Sequence Auto", where only one can be selected. When "Sequence Auto" is checked PM would use the existing {seqn} command and the "sequence value" entered to begin sequence numbering with the next available number. Again, PM looks at existing filename's for a sequence number that is in the location and place value indicated in the renaming string.
An example renaming string would be: {yr4}-{mn0}-{day0}_{seqna} with a sequence variable value of 0001, which in this case PM uses only used to determine the number places to consider.
I hope this makes sense, please advise if you have any questions.