I don't see anything that prevents the use of a pad or piece of cloth from being used in either IBS or UBS rules. The following is a quote from the IBS rules. Maybe I am not reading it right.
H) Rests: 1) A front rest shall support the front part of a rifle: a rear rest shall support the rear part of a rifle; neither rest may be attached to the bench, the rifle, or to the other (exception - Heavy Bench rests may be one piece for front and rear). 2) Unrestricted rests may incorporate guiding means, and adjustments for elevation and windage in either or both components. Unrestricted rests may be of one-piece construction for front and rear. BOLD RED TEXT SIGNIFIES 2010, 2011 RULE CHANGES. II. Definitions 3 II. Definitions 3) The rear sandbag shall be a bag, or combination of bags containing sand only. Zirconium silicate, known as “heavy sand”, is allowed. No other metallic sand will be used by any competitor. A vertical spacer under the rear bag will be allowed as long as it incorporates no adjustments for windage or elevation. The vertical spacer shall not contain any protrusions, which can be inserted into the bench top or the sandbag. The rear sandbag shall not be contained in any manner. The rear sandbag cannot in any way attach to the front rest. 4) The front bag must be such that the rifle can be easily removed in the vertical direction without any front sand bag adjustment. The bag must also be designed to maintain a 100% contact with the bottom of the fore-end when in its shooting position. The cover and filling of the front bag shall meet the requirements described in paragraph (3) concerning the rear bags. The front bag shall also be a minimum of one half inch thick in the vertical direction within the width of the fore-end. I) Guiding Means: Any device, addition, contour or dimension on any rifle of any class, designed or adapted to co-act with mating or reciprocal features of a rest to guide its return to firing position so that it shall not be necessary to re-aim the rifle optically for each shot shall be conclusively deemed to constitute guiding means.