Not to sound too harsh about it but why bother? Let the devs do what they want. If they want a game to rely on the HDD then say on the box that its required. Theres no point walking on eggshells around the people without HDDs. It was a mistake not having them in all SKUs from the start so why not rectify it? If they bring out a 60gb there will be a lot more 20gbs preowned on sale and everyone can get one. Mines cost me £50 and that was long before the 120 came out, I dont think it'll cost much to get one now, and it'll just be the cost of keeping up to date.
Lets face it, we've all had to upgrade PCs time and again for some game... will a single payment to get full use of the 360 really bother that many?