I don't think that really works for a top 20, and I have voted against top 20 witnesses who I saw trying to treat it as such. Given that, I wouldn't feel right doing so myself. You need to engage on the platform, be around for emergencies, and work regularly with other witnesses, developers and stakeholders on planning and decision making.

It is possible I may run a backup witness at some point, though most of the purpose for backup witnesses is to rise up in the ranks and then (in the 20-30 ranks) be prepared to serve as top 20 if needed. So it would still take a change in my availability to make sense. That may happen someday, but not now.