In order to use python code (in general) within an RML-file, you have to add that method to the localcontext of the report.

Example

The following code I have as sale_order.py in the report-folder of my custom module, where the custom report is as well.

from sale.report import sale_order
from report import report_sxw

# create a custom parser inherited from sale order parser:
class new_order_parser(sale_order.order):
    '''Custom parser with an additional method
    '''
    def __init__(self, cr, uid, name, context):
        super(new_order_parser, self).__init__(cr, uid, name, context=context)
        self.localcontext.update({
            'has_line_discount': self._has_line_discount,
        })
        self.localcontext.update({
            'show_discount':self._show_discount,
            'payment_term':self._payment_term,
        })

    def _has_line_discount(self, order):
        return any([l.discount for l in order.order_line])

    def _payment_term(self, order):
        return _(order.payment_term.name)


# remove previous sale.report service :
from netsvc import Service
del Service._services['report.sale.order']

# register the new report service :
report_sxw.report_sxw(
   'report.sale.order',
   'sale.order', #model to use
   'path/to/new/report.rml',
   parser=new_order_parser
)

To use the custom report, I import the above fill in __init__.py (also in the same report-directory):

import sale_order

in the file __init__.py at the root of my custom module, I have the line:

import report

The only issue I run into is that I have to specify manually within openERP where my report.rml is located.