If, like Joe, I wanted to keep to a 3 x weekly schedule but felt that I couldn't do a particular body part justice or to try to 'bring up' a weak part, I'd do a 2 x weekly full-body and then use the other session to focus on the particular body part or a specific lift that I wanted to improve. It should be possible to do enough volume for the lagging part this way whilst still getting plenty of work done for the rest of the body.
Another approach would be to stick with 3 x weekly fullbody w/os and then to use a major compound for your lagging body part first. So, if pecs needed bringing up, you might do some form of bench or dips first when you are freshest, perhaps adding a little extra volume too. Do that for a cycle and then focus on something else.
Personally, I couldn't give a monkey's what muscle-this-that-or-the-other.com thinks HST is. A lot of folks visit the HST site while doing a bit of personal research, perhaps because they want to better understand how the body responds to training. Others are happy to take what someone else tells them to do, do it until it stops working, and then try the next 'new' thing. If they stick at it long enough and keep adding weight to the bar over time, they may even get quite good results.
It seems to me that most successful lifters (ie. those who attain their goals) are the ones that keep on plugging away year after year, keeping track of everything they do, finding out what does and what doesn't work for them and changing things up accordingly. The fundamental principles of training for hypertrophy and strength won't have changed over their whole 'career' so they'll most likely end up with routines that will look a lot like an HST or an SST routine, using movements that fit with their particular goals (eg. powerlifters will be focussing on the three competition lifts and adding in ancillary work).
For me, what's really good about HST as a way to train is that I know exactly what I'm going to lift each time I hit the gym; it's all planned out in advance. There is no "Now then, what shall I do today?" I always want to make my lifts because I know I should be able to get them. The only days that I would be unsure about would be days where I push for new PRs - usually at the end of a cycle. This is very different to what a lot of new trainees will be used to if they've come from a 'put the load up when you can get X reps' background; but it will be very familiar to those who come from a background of power training where load and intensity levels are often cycled and carefully planned out in advance to aid peeking for a meet.
So, for me HST is still very relevant. All the principles can successfully be applied to a new lifter's, or experienced lifter's, programming. I don't view HST as a 3 x weekly full-body routine because it isn't, but even if you use it that way you should still be able to make some good progress until you are close to your GP. It's always a good idea to see what a similarly sized, drug-free lifter might expect to achieve by checking this site:
http://www.weightrainer.net/bodypred.html
