So I implemented (thank  for a suggestion in Emacs-ru chat) optional
Ibuffer viewer. Diff on project.el is attached. It works in my setting.

Best wishes, 
-Mikhail

On Sat, 2024-06-01 at 23:07 +0700, mikpom wrote:
> Thanks for pointing to ibuffer-project. It will deliver on my request
> even if suggested feature is never implemented.
> 
> On Sat, 2024-06-01 at 18:27 +0300, Dmitry Gutov wrote:
> > Hi!
> > 
> > On 31/05/2024 15:28, mikpom wrote:
> > > Greetings devs and emacs community.
> > > 
> > > I occasionally need to list buffers of a project and do so with
> > > the
> > > built-in command project-list-buffers from project.el. The
> > > problem
> > > is
> > > that this command utilizes Buffer-menu mode and does so in a
> > > hard-
> > > coded
> > > way. Personally I prefer IBuffer which is already heavily
> > > customized
> > > while I have zero knowledge of Buffer-menu mode.
> > > 
> > > I suggest to make an option whether project.el will list buffers
> > > with
> > > Ibuffer, Buffer-menu or even something else?
> > 
> > I'm happy to add a new option, but if you look at the definition of
> > project-list-buffers, it's not as trivial as
> > 
> >   - bind default-directory
> >   - call function
> > 
> > And ibuffer seems to require some effort as well. As a consequence,
> > the 
> > option is unlikely to be infinitely extensible, it would just list 
> > 'list-buffers' and 'ibuffers' as possible values. Maybe a "Custom 
> > function" alternative too, but the users who would want to use it
> > will 
> > need to put it some work as well.
> > 
> > Speaking of ibuffer in particular, here's a hint of setup required
> > to
> > use it for project buffers: 
> > https://github.com/emacsmirror/ibuffer-project/blob/master/ibuffer-project.el
> > 
> > Of course, a lot of this could be optional (improvements to make
> > the
> > UI 
> > better), so if someone wanted to contribute a minimal
> > implementation,
> > we 
> > could certainly use it (just please don't copy the code from the
> > above, 
> > it's not copyright-assigned).
>