I found the correlation between HR frequency and HBP frequency for each decade since the 1950s. In one case the denominator was AB + HBP, in the other it was AB + BB + HBP. Here are the correlations for the first case, starting with the 1950s
Now for the second measure.
The correlations are higher in the 80s, 90s and the 2000s, meaning players who hit HRs more frequently are more likely to get hit by a pitch than in the the 50s, 60s and 70s. So when old-timers tell you something like "if you hit a HR off Bob Gibson, next time you got brushed back or put on your but," don't believe them. If that kind of thing was going so much, there would have been more hit batters (some of those brushbacks would be a little off the mark, so the pitch would hit you, not just come close). And the correlation would have been higher between HR hitting and getting hit back in those days. But they are higher now.
In fact, hitting a HR in the 1990s increased your chances alot more than hitting a HR in the 1960s. Here is the regression equation from the 1960s
HBP% = 0.0311*HR% + 0.0058
Now for the 1990s
HBP% = 0.0573*HR% + 0.0065
Since .0573/.0311 = 1.83, hitting a HR in the 1990s was 83% more dangerous in the 1990s than it was in the 1960s. And the T-value on HR% in the 1990s was significant (2.84) while it was not significant in the 1960s (1.52).