It could definitely be done as long as you identify the ct. Gel vs qPCR doesn't matter.

Cheers!

If you are in the linear range and can measure fluorescence of EtBr bands (not densitometry) e.g. in a Licor Odyssey, I would say you could use ddCT method to compare expression. Since EtBr intercalation (mean: brightness of band) to DNA is proportional to fragment size (bp) and mol of fragment, you should correct fro this comparing amplicons of different size (sort of normalization). You also will have to assume that amplification efficency of different  primer pairs is similar. It definitely will be much dirtyer than qPCR. but to get an inital idea... why not?

I would not recommend it. As far as I know gel densitometry is too much depending on gel thickness, EtBR distribution, precise cut etc. - it is similar to a Western Blot in terms of quantification which means ... roughly quantifiable but not very sensitive. A qPCR is very likely way more sensitive and precise than that.

Thanks for the answers. You really confirmed my suspicions. Normally I would do this by qPCR and the reasons for this are unequivocal. However at the moment we are having access problems to qPCR platforms so I was look for a rough and ready (by comparison) temporary alternative

qPCR was dreamt up by Higuchi using the very idea you are proposing here. (This is how qPCR was invented!).  So, it is worth a try - with caution though - as you and others have mentioned.

I guess five or more rxns per sample are need it, perhaps every 2-3 cycles between 22-32 cycles. But my guess is that it would be to expensive. Why not competivitive PCR with purified PCR products?

First, delta delta CT is a method which can beter replaced by LinRegPCR and caluclation of N0. The latter method corrects for differences in efficiency between various genes as well as for exreiment-contyrol conditions.

Second, if you read MIQE, the terms housekeeping genes should be replaced by reference gene and CT must be replacerd by Cq.

Third. Why would you replace a quantitative RT-qPCR signal by densidometry? qPCR is measured at the log- linear phase of an indiviual amplification curve. In my opinion you calculate density at the pleateau phase after {PCR. Then its impossible to correct for input parameters such as initial target concentration. Morover, if you perfoirm the qPCR in a correct way, the plateau phase is the same whatever input concentration of targets you use (need to perform 50 cycles).