I've never made that particular connection, but I have needed to make tight connections in the opposite direction after arriving at St. Pancras. As Alfonso says, you can walk up the train even if unable to amend your seat reservation - I've done this more than once (moving all the way from my second class seat through the first class section to the front of the train) and didn't get anything but a few bemused looks from other passengers; most didn't even notice. Furthermore, there are no at-seat ticket inspections on the Eurostar and the train is often half empty when I travel, so I personally wouldn't have any qualms with simply sitting in a different seat of the class I booked in, regardless of the seat reservation, and just standing for the remaining twenty minutes if someone gets on at Lille and wants the seat.
For tony6499, you are well-protected in the (disappointingly likely!) case that the Eurostar is late. As you are travelling with Eurostar and Deutsche Bahn, you have both the Agreement on Journey Continuation and the Hop On The Next Available Train scheme covering this situation. Just mention the Eurostar delay to the train manager or conductor and I'm sure you'll be fine; they are always very amenable in my experience.