Just in case it might be interesting for some of you:
After having chatted with the Citibank CS guy I found out that it is possible to 'indirectly' settle IRD bills with the Cathay co-branded CC. There is no way to settle it directly, but you can apply for a loan issued as a cheque. It's even possible to apply for it online by using the Citibank website to login to your CC account ("Click-A-Check"). With that cheque you can then pay the IRD.
But obviously a loan is always inevitably linked with interest rates and the best off-the-shelf interest rate is something like 1.98% per month. There is a low interest rate promotion with around 0.59% per month, but this will only be available to a few selected CC holders (most probably for those with a very high spending anyway). Maybe even an extra handling fee will apply if you pay back the loan earlier than the minimum loan period of 6 months.
They specifically told me that even if I would have enough money on the CC account, I would still need to apply for that kind of loan. I assume it's similar like the inevitable Cash Advance fee. And as mentioned by PDLM already, no Asiamiles will be credited for transactions to the IRD in any case.
That's not really interesting for me, as I rather prefer to pay my taxes without paying any interest. It may only make sense for some to easier reach an annual spending of more than HKD 150,000 to get the annual fee waiver of HKD 1,800 for a Cathay Platinum Select Preferred card. It has been confirmed that IRD payments will at least be accounted into the annual CC spending to determine the annual fee waiver eligibility. The break-even for 1.98% per month and an assumed HKD 500 handling fee for early re-payment of the loan would be at HKD 65,657 or less tax payment per year and for 0.59% at HKD 220339 or less if you settle your CC bill immediately. The latter one should be sufficient to all those ones who are interested in collecting Asiamiles at all, as the taxable annual income for that HKD 220K tax amount would be somewhere above HKD 1,37M