How can a subcontractor force the GC and owner to finalize a change order?

I am a subcontractor providing manufactured casework and architectural millwork for a new veterinary hospital in Wisconsin. In August, after the architect/GC approved shop drawings, the owner requested many scope changes. We submitted the cost impact of those changes as a change order request a couple weeks later. Those changes were followed by revised shop drawings including the new scope, and those drawings were subsequently approved. Since that time, the owner has asked several rounds of questions about the costs, but also revised their request to reduce total cost...acknowledging that their actions had cost implications. The job went into production in November, without an approved change order. At the GC's direction, we were asked to meet the schedule and that the customer was committed to the expanded scope and that they would approve the change order. After many promises to issue the change order, we still do not have that in place. The PM recently expressed concern about whether they would even get paid, but still said he would send the change order. Now the job is nearing an end and the absence of that document prevents me from sending an invoice for the total scope. Last week I met with the leadership of the GC and had a very friendly and professional conversation about the issue. I think they were quite concerned that their PM didn't resolve this over the last five months and said they would investigate...though stopped short of promising to resolve to proper satisfaction. The real question is whether or not I can use the process leading up to a Mechanic's Lien? There isn't an actual contractual obligation to pay me without the change order. So can I use a Demand for Payment letter? And can that be followed by a Notice of Intent to file a lien, with the Mechanic's lien to follow if necessary? If not, what other tools do I have for remedy if the owner continues to stonewall the GC?