In the US, are inside loops permitted while flying a US- registered DG-1000S sailplane in the "utility" category? 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: 
  
  Spins 
  
  Chandelle 
  
  Inside loop 
  
  Turn 
  
  Lazy Eight
  
  Recommended entry speeds see section 4.5.8.1.


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: 
  
  Inverted flight  
  
  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 
  
  slow roll


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.
Stuff you quoted in your question seems to provide the answer you're after:

2.6 Approved manoeuvres Category „Utility“: The glider is certified for normal gliding in the &quot;Utility&quot; 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:
...
Inside loop