You need to create new model or override an existing model and add the method there. After that you can use the self.env['module_name'].whichCompanyPrefix

This works too. I hope it is ok too.

In the file /Users/Flooder/.mount-odoo-test/addons/report_lapagept/controllers/control_pt.py :

def whichCompanyPrefix(self):
        self.cr, self.uid, self.pool = request.cr, request.uid, request.registry
        user = self.pool["res.users"].browse(self.cr, self.uid, self.uid)
        company_id = user.company_id.id
        company_name = user.company_id.name
        company_prefix = user.company_id.x_company_prefix
        if not company_prefix :
            company_prefix = 'ZZ'
        return company_prefix

In the file /Users/Flooder/.mount-odoo-test/addons/report_lapagept/controllers/main_pt.py, I could call whichCompanyPrefix like this :

from openerp.addons.report_lapagept.controllers.control_pt import whichCompanyPrefix
class PTPosController(PosController):

    def a(self, debug=False, **k):

        prefix = whichCompanyPrefix(self)

May be it could help somebody.