Gunners have never been constrained in the use of their field pieces by engineering/manufacturing specifications. Fixed max elevation on 18 Pounders and 25 pounders were overcame by digging in the trails (
http://books.google.ca/books?id=oLge...g%20in&f=false see image at page 38). This changed the maximum ordinance or max height of a round and its overall range. The data for exceeding max elevation could not be found in its firing tables, but the fall of shot would be consistant making adjustment of artillery fire at high angle a reality.
Modern day howitzers (towed or tracked) have a greater ability to engage targets in "high angle" or in the "upper register" than their predecessors, but if you absolutley had to engage a target close to the gun position but hidden behind a high obstacle, you need only to request fire from another call sign (arty, armoured, attack helicopter or if they check out of the Holiday Inn on time, fast air support) that can reach out and touch the target.