It has always been illegal to cross the border into the US where there is no official border crossing station. The law has not changed. The only difference is that recently the Border Patrol is making more noise about it. I have no idea, though, if this means increased patrols or just more bureaucratic noise. In the past, most people have just done it, quietly. EDIT, LATER: I am not advocating this postition!
If you don't want to be illegal, you'll have to hike from Harts Pass or Rainy Pass to the border and back again. Nobo's who can't enter Canada due to a past DUI or other infraction have to hike out this way.
Another possibility is to hike the East Bank Trail or take the Ross Lake Resort water taxi along Ross Lake to Lightning Camp and take the Three Fools Trail up to the PCT. This could take longer than an up-and-back on the PCT, especially since trip reports from the Three Fools Trail the past few years say that it is no longer maintained--lots of brush and deadfall. It would certainly be an adventure, though!
Why the US can't set up an "apply for a permit in advance" system like Canada's for PCT hikers is a mystery to me.