Are inside loops are permitted while flying a US-registered DG-1000S sailplane in the US in the utility category?
FAR 91.1 seems highly relevant:
§91.9 Civil aircraft flight manual, marking, and placard requirements.
(a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.
More information: the English version of the factory-produced flight manual for the German-made DG-1000S glider (may be downloaded here) states the following--
2.6 Approved manoeuvres Category „Utility“: The glider is certified for normal gliding in the "Utility" category. Simple aerobatics are approved but only without waterballast and with the weight of the rear pilot compensated by ballast in the ballast box in the fin see section 6.8.7.
The following aerobatic manoeuvres are approved with all spans:
Recommended entry speeds see section 18.104.22.168.
The manual then goes on to describe additional maneuvers allowed when flown at a lower weight limit, in the "aerobatic" category:
In addition to the manoeuvres in category „Utility“ the following manoeuvres are approved:
half flick roll from normal to inverted flight with half loop
half loop and half roll
half flick roll from inverted to normal flight
half roll and half loop
Before answering, you may wish to consider the significance of the last quoted phrase in FAR 91.9, "or as otherwise prescribed by the certificating authority of the country of registry". A "yes" answer to the basic question implies that this phrase in FAR 91.9 does not somehow prohibit looping a US-registered example of this glider in the US.