Hi Marc,
It is indeed possible to perform absolute flux calibration of spectra using photometric magnitude measurements as a reference (as opposed to just calibrating the spectra relative to the continuum). It is a technique frequently used by amateurs these days to calibrate broad band (typically low resolution) spectra where an absolute flux calibration is required. The details of the method were developed on the ARAS forum. Tools were built into ISIS software and the procedure was formalised by BAA member here, David Boyd.
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=897
and has been used in several of his (peer reviewed) papers in the BAA Journal.
It is not strictly valid to use the technique in the way Ernst has done here however as the spectral range covered is not the same as that of the V filter passband The implicit assumption here is that the flux in the V band and in the continuum at H alpha are linearly correlated. This is not always the case and would need to be established, for example by performing simultaneous photometry in V and R bands.
Alternatively more conventional techniques for absolute flux calibration of spectra are also used by amateurs by recording spectra of spectrophotometric standard stars, aided for example by the use of dual width “spectrophotometric” slits available for the Shelyak spectrographs which allow the total flux to be collected. This does need stable atmospheric conditions however during the observation. (analagous to all sky photometry). Again ISIS software has the tools to enable this.
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=695
Cheers
Robin