When inheriting account.tax, you can overwrite the "name_get" method. You can use that method to make Odoo return a custom value for each time the record appears somewhere on a list or form. 

Note: This is system-wide, so ALL the occurences of account.tax will use this method to determine the display-name.

This is an example as found in the standard account module:

    def name_get(self, cr, uid, ids, context=None):
        if not ids:
            return []
        types = {
                'out_invoice': _('Invoice'),
                'in_invoice': _('Supplier Invoice'),
                'out_refund': _('Refund'),
                'in_refund': _('Supplier Refund'),
                }
        return [(r['id'], '%s %s' % (r['number'] or types[r['type']], r['name'] or '')) for r in self.read(cr, uid, ids, ['type', 'number', 'name'], context, load='_classic_write')]

Ludo, it seems that I cannot comment directly to your post, probably due to lack of 'reputation' or something like that. Thanks for your answer, however, how can I apply that change on the odoo.com online service?