Not for OpenBSD, maybe, but for other operating systems based on precompiled binary packages. Nginx uses a forked worker model that probably would allow adding/removing modules without restarting the entire server.
Considering that you can upgrade the binary without any downtime then yeah, it probably would. I personally think there are bigger issues that needs to be covered first, such as support for persistent connections to backends and HTTP/1.1 support when reverse proxying. (Was just added in 1.1.4, now it just needs testing!)