I know this question is a bit old, but I thought I'd answer for the googlers out there.
You could use this project. Its basically a javascript interepter that has access to the .Net framework.
So you could do something like:
jish.assembly('path/to/System.Windows.Forms.dll');
var mb = jish.create('System.Windows.Forms.MessageBox');
mb.Show('Hello World');
And it works, I've not however tried more complex winforms apps so can't say whether it will fall down eventually.
Let me know if anyone tries it.
Edit 1: Well I tried it with a slightly more complex example and it worked also. Try this:
jish.assembly('path/to/System.Drawing.dll')
jish.assembly('path/to/System.Windows.Forms.dll')
var app = jish.create('System.Windows.Forms.Application');
var form = jish.create('System.Windows.Forms.Form');
var lbl = jish.create('System.Windows.Forms.Label');
form.Text = lbl.Text = 'Hello World!';
lbl.Location = jish.create('System.Drawing.Point', 50, 50);
form.Controls.Add(lbl);
app.Run(form);
Guido