File updated - a "couple seasons" turned into doing 1899, 1903, '08, '13, '17, '19, '21, '23, '31 and '48. And this turned up a new peak - 57% in that deadest of deadball seasons, 1908.
I haven't found very much beyond that SABR article to cross-check deadball-era B-R HR log data against, but from what I do have it seems to measure up well. A
1978 SABR article says 34 of 38 homers in Braves Field in '21 were ITP, and that's what I have as well.
This book says there were 20 inside-the-park home runs in Yankee Stadium in its first year, but I counted 29.
This very extensive write-up on Hilltop Park lists sixteen home runs being hit there in 1903, 1 of them inside the park, which agrees with what I found also. A few home runs are missing from the log however, as I remember seeing some cases where a player has one fewer home run in his log than his official total.
I don't have the time now to post the rest of what I was going to post, so it will follow soon.