Had PAF gone for ukrainian flankers, they could potentially have been supplied with parts or even upgraded by China, although that might have pissed off the Russians, and China wanted to sell its own planes so it would have been foolish for them to agree.
But if PAF did buy it, there is a chance that China could have supplied parts using unofficial channels so they could plausably deny any involvment and the PAF could have claimed that they made the parts themselves.
If PAF desperately wanted flankers, it was certainly a viable option. But they probably didn't want them all that much on account of how old those flankers were. There can't have been that many hours left in the airframe, and you would need some serious work done to upgrade them to have any chance against the MKI. Definately nit worth it.
If PAF just wanted aggressor training, they could have flown against Chinese MKKs and/or J11s.
The PAF might not be able to get F16s into China without risk of America finding out, but if they are flying JF17s, who's to tell who is in the cockpit if the participants keep quiet?
I see a lot of potential in such exchanges since the PAF gets first hand experience fighting flankers, and the PLAAF gets to fly against NATO standard pilots to expend their knowledge and keep up to date with current western tactics and doctorines. Just don't expect to read about it if everyone involved is doing their job properly.