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Actually it could work but it needed some workaround for the moment.

I'll explain first how sockets and plugs work (to my understanding)
Basically, each socket connect to others sequentially.
for example we connect Socket A -> Socket B
if socket A has 1 plug, and socket B has 2 or more plugs, then only A:1 -> B:1 is connected. Notice the 'Click track from ardour' connection to 'PA Audio' in the screenshot from the article.
if socket A has more then 1 plug (for example 4) while socket B has less plugs than A (for example only 2) then the connections are A:1->B:1, A:2->B:2, while A:3 and A:4 is unconnected.

Now for your need to connect a mono output port to a stereo input ports you need to have
Socket A:
+plug 1 = mono_synth_out
+plug 2 = mono_synth_out
Socket B:
+plug 1 = stereo_input_L
+plug 2 = stereo_input_R

if you connect A->B, then mono_synth_out will be connected to stereo_input_L and stereo_input_R, just like you wanted.

The problem is that QJackCtl's Patchbay interface actually removed the ports from the list of available plugs to add if you already add the port.

Take a look at this screenshot http://imagebin.org/260685

It produces this XML code in the patchbay save file.
<socket exclusive="off" client="zita-mu1" type="jack-audio" name="Output Socket 4">
<plug>sel_out\.R</plug>
<plug>sel_out\.R</plug>
</socket>

In order to achieve that, I add two plugs, sel_out.L and sel_out.R, and then renamed sel_out.L into sel_out.R.
But for a monosynth it's impossible to achieve that because once the port is added as a plug then the list of ports is empty.
There might be a workaround though.
For example you have these clients 'MonoSynth' ('mono_out') and 'system' ('capture_1' and 'capture_2').
Create Output Socket Stereo_Synth.
Choose 'system' as the client, add capture_1 as the first plug.
Change client into 'MonoSynth' and add mono_out as the second plug.
Rename the first plug into mono_out.

Now, Rui's suggestion to make two Mono Input Sockets also work, but it means having two connections MonoSynth->Input_L and MonoSynth->Input_R.
if you have a lot of stereo clients or already make stereo sockets, then that means having duplicate sockets for the mono connections.

This actually means that it depends on your workflow.
if you work with stereo connections then the majority of your sockets would be stereo.
I find that using QJackCtl's Patchbay simplifies connecting the ports by grouping the ports into stereo sockets.

So, I have two suggestions for Rui regarding this workflow.
1, make the list of ports in the Socket window to always shows all available ports in the system. (the connection already works, it's just the UI does not let the user to add them)
2, if it's possible to make that each socket can have plugs from multiple clients (I think this would be harder, from the patchbay xml format it seems that you coded each socket to be tied to a certain client.)
Perhaps something like this
<socket exclusive="off" type="jack-audio" name="Output Socket 4">
<plug>zita_mu1:sel_out\.R</plug>
<plug>system:capture_1</plug>
</socket>
or
<socket exclusive="off" type="jack-audio" name="Output Socket 4">
<client name="zita_mu1">
<plug>sel_out\.R</plug>
</client>
<client name="system">
<plug>capture_1</plug>
</client>
</socket>

Perhaps I'll take a look at the source code sometime later if I could hack on them (but probably will take a looong time, I'm still learning how to code, and QJackCtl's code is pretty big -- wc shows >30K lines)