Basically they just have to lend it tacit support so money can get in an out, especially or mainly USD
The only thing this guarantees is that they lose control over their own money getting in and out.
I think the strategy you're suggesting is that the Chinese will allow Bitcoin to be adopted, possibly even go as far as to promote it. Then they'll hope that Bitcoin does well. Then they'll hope that US citizens will be encouraged by that success alone and adopt Bitcoin as their main form of currency. Then they hope they'll transfer that wealth elsewhere. All the while hoping that Bitcoin does collapse for a myriad reasons.
Seems an unlikely and unreliable strategy to me and a very high risk one at that.
There's a much better chance that they'll just lose control of their own currency than they will reduce Dollar hegemony by any significant amount.
There are lots of more effective and reliable ways of reducing Dollar hegemony that don't require giving their citizens fuller and less transparent control to their own wealth.
The CNY govt has been very good an controling RMB value
all they have to do is set up some where, or multiple places, where USD can in volume, easily and reliably be traded for BTC.
This wont let RMB in easily, they can crack down on that, and USD not really able to be used in CNY
But I see your point.