As a reference, here is the KB describing the custom code that is allowed for use in Microsoft Dynamics CRM Online FetchXML Reports:

http://support.microsoft.com/kb/2600639/en-us

When RDL Sandboxing is enabled, only certain classes and methods can be used in a custom FetchXML report. The following is a list of the classes that are allowed, and beneath each class are the available methods for use in Microsoft Dynamics CRM Online reports:

1.Microsoft.VisualBasic.Interaction (For more information, see: Interaction)

• IIF
• Partition
• ToString

2.Microsoft.VisualBasic.Information (For more information, see: Information)

• IsArray
• IsDate
• IsNothing
• IsNumeric
• IsReference
• QBColor
• RGB
• ToString
• TypeName
• VarType

3.Microsoft.VisualBasic.Strings (For more information, see: Strings)

• Asc
• AscW
• Chr
• ChrW
• Format
• FormatCurrency
• FormatDateTime
• FormatNumber
• FormatPercent
• InStr
• InStrRev
• Join
• Lcase
• Left
• Len
• LTrim
• Mid
• Replace
• Right
• RTrim
• Space
• Split
• StrComp
• StrReverse
• ToString
• Trim
• UCase

4.Microsoft.VisualBasic.DateInterval  (For more information, see: DateInterval)

• Year
• Quarter
• Month
• Day
• WeekOfYear
• WeekDay
• Hour
• Minute
• Second

5.Microsoft.VisualBasic.DateAndTime (For more information, see: DateAndTime)

• DateAdd
• DateDiff
• DateValue
• Day
• Hour
• Minute
• Month
• MonthName
• Second
• DatePart
• DateSerial
• TimeSerial
• TimeValue
• ToString
• Weekday
• WeekdayName
• Year

6.Microsoft.VisualBasic.Financial (For more information, see: Financial)

• DDB
• FV
• IPmt
• NPer
• Pmt
• PPmt
• PV
• Rate
• SLN
• SYD
• ToString

7.Microsoft.VisualBasic.Conversion (For more information, see: Conversion)

• CTypeDynamic
• Hex
• Oct
• Str
• ToString
• Val

8.System.DbNull (For more information, see: DBNull)

• GetObjectData
• ToString

9.System.Globalization.CultureInfo (For more information, see: CultureInfo)

• ClearCachedData
• Clone
• CreateSpecificCulture
• GetConsoleFallbackUICulture
• GetCultureInfo
• GetCultureInfoByLeftLanguageTag
• GetCultures
• GetFormat
• ReadOnly
• ToString

10.System.Math (For more information, see: Math)

• Abs
• Atan
• Cos
• Exp
• Floor
• Log
• Log10
• Max
• Min
• Round
• Sign
• Sin
• Sqrt
• Tan

11.System.String (For more information, see: String)

• Clone
• CompareOrdinal
• Concat
• Contains
• Copy
• CopyTo
• EndsWith
• Format
• GetEnumerator
• IndexOf
• IndexOfAny
• Insert
• Intern
• IsInterned
• IsNormalized
• IsNullOrEmpty
• IsNullOrWhiteSpace
• Join
• LastIndexOf
• LastIndexOfAny
• Normalize
• PadLeft
• PadRight
• Split
• StartsWith
• Substring
• ToCharArray
• ToLower
• ToLowerInvariant
• ToString
• ToUpper
• ToUpperInvariant
• Trim
• TrimEnd
• TrimStart

12.System.Text.RegularExpressions.Match (For more information, see: Match)

• ToString

13.System.Text.RegularExpressions.Regex (For more information, see: Regex)

• Match
• Replace
• Split
• ToString