The HK airport website, which seems to be generally accurate on these things, shows the United Club to be "Temporarily closed until further notice".

The site also lists all the available corporate/pay-in lounges. You'd need an Amex expert, which I am not, to answer your question about eligibility there. The Plaza Premium lounges don't generally get great reviews.

There is also the "VIP Lounge" which costs HK$5000 for a 3-hour stay.