Apache MPM event looks like the answer, but it's experimental and has some key modules are missing yet.
I did some networking with people working on the project. I learned that the statement in their docs about no support for mod_ssl and most other input filters date back to the 2005 release of Apache 2.2. That is no longer the case today. Apparently mod_ssl and mod_rewrite and everything else has been working for quite awhile. I queried for the issues and noted that almost every one has a patch available. Released software has at least that many bugs as I see here:
https://issues.apache.org/bugzilla/...me&field0-0-0=noop&type0-0-0=noop&value0-0-0=
It may be exciting to think that we could use Nginx or Lighttpd with DirectAdmin, but from a shared hosting perspective, it would be an untenable situation to limit people to the small cross section of features non-Apache servers provide. We like to think we don't use a lot of Apache's capabilities, but you can't even do a decent captcha on a contact form without them, and who knows what else we would reach for and not find. Neither us nor web hosting customers are accustomed to thinking in terms of working around web server limitations when deploying web apps. So then how many DirectAdmin customers could actually use it?
I haven't tried MPM Event yet. I've read the comments of a few people who use it. According to them, they were never able to make MPM Worker fast nor reliable while MPM Event just works and is fast. The MPM Event option provides all of the capabilities of Apache plus the event-based front end of an Nginx or Lighttpd. The next Apache, 2.4, adds to that small-object caching and a memcache distributed cache.
The need for specialty servers has been shrinking steadily since Apache 2.x came out, and especially 2.2 back in 2005. Running PHP under Fastcgi is not a performance advantage, it is a performance limitation required to make up for one of the many limitations of these servers. Netcraft statistics confirm they have ceded the dynamic ground to Apache, and now, with an Apache event-driven front end, the ground specialty servers will occupy will be reduced yet again. Their only remaining bastion will be clustered environments where separate physical servers handle static content. Very few of those servers will be running DirectAdmin. I'm all for adding another server if it makes sense for the future. The way it appears to me, what is being proposed only makes sense for the past, and maybe not even the present.
Adding MPM Event to custombuild would be a cake-walk, plus DA and everyone else is already very familiar with Apache and it's modules. For a non-threaded Prefork-based environment with user based security advantages of FastCGI, there are extensions such as MPM ITK, MPM PerUser, etc.