Why is lowering pH in secondary a bad thing? Almost any fruit would do this, as most are high in some combination of citric, malic or tartaric acid. As a matter of fact, I find that if the pH is too high, then that leads to an insipid fruit beer. A touch of acidity (often coming from the fruit itself) is enough to lift up the fruit character a bit and helps with the end product, in my opinion.
I will acknowledge that I do not have enough knowledge to offer up an, lets call it proper, opinion. But back to the OP the target beer is a Tripel with a starting pH of 5.2-5.4? Adding something like 1.2g (from his 100g of rosehips) of ascorbic acid after primary fermentation when pH has dropped to something like 3.8-4.4 wouldn't cause a problem in the final pH of the beer?
Quote from the Principles of Ph from byo.com
And Finally, Fermentation
During fermentation, the pH continues to drop for a variety of reasons. Yeast cells take in ammonium ions (which are strongly basic) and excrete organic acids (including lactic acid). The yeast strain chosen can affect the final beer pH. Most lager beers finish at 4.2–4.6, with some ales ending as low as 3.8. (Sour beers may have pH values around 3.0.)
Achieving an optimal pH, less than 4.4, favors faster beer maturation (including uptake of diacetyl), better beer clarity, better biological stability and a "more refined" beer taste.
Brewers rarely adjust final beer pH with acid. To reach a suitable final pH, all that is needed is to conduct a good, vigorous fermentation. As pH decreases with attenuation, drier beers tend to have slightly lower pH values. One interesting tidbit about fermentation is that some molecules in the fermenting beer become decolorized as the pH lowers and so the color of beer actually lightens slighty during fermentation.http://byo.com/malt/item/1494-the-principles-of-ph
I took the quote "Brewers rarely adjust final pH with acid." As a sign to not add an acid that would cause the pH to lower, if that was not desired. Which if you lowered a Tripels pH with 1.2g of ascorbic acid would it not drop the pH into a sour range? I would assume that to be undesirable.