Because of the wicked thunderstorms in the northeast US, I was alternately without power or cable/internet for several hours and was unable to post my lowest score of 326. I've included those enhancements as comments in the code, marked by "LATE! -# strokes". Damn weather made us postpone my daughter's 12th birthday party today!

Somebody asked for commented code so they could learn about the logic, so, here's my commented 337-stroke version:
Code:

; begin Mayan Number Converter
;
;!
Function m($) ; - 57 stroke overhead

Dim $_,$C,$D

If VarType($)=8 ; is it a mayan string?

; Mayan to Decimal - 96 strokes
 $D=1		; initial multiplier value
 
 ; work from LSD to MSD
 For $_=Len($) to 1 Step -1

  ; get Mayan char value based on string position
  ; InString will report 1, 2, 5, or 6 (delimiter)
  $C=InStr('.:xx| ',SubStr($,$_,1))

  If $C=6 	; Delimiter? move to next multiplier power of 20
   $D=$D*20
  Else		; is a vaid digit value - multipliy by current multiplier value
		; and add to running total
   $m=$m+$C*$D ; process value
  EndIf

 Next ; next string character

Else ; or a Decimal Value?

 ; Decimal to Mayan - 184 strokes
 $D=64000000	; highest divisor value for 7 digits

 While $D>=1	; loop while divisor is greater than 0
 ;While $D	; LATE! -3 strokes
  ; divide by max value, result will be between 0 and 19 - a Mayan "Digit"
  $C=$/$D $=$ Mod $D

  ; If not zero, output the Mayan chars that make up this "digit"
  If $C
   $m=$m+$_+SubStr('.',1,$C Mod 5 Mod 2)+SubStr('::',1,$C Mod 5/2)+SubStr('|||',1,$C/5)
   $_=' ' ; inter-digit space after the first one
  Else
   ; if zero, output a zero only if other digits exist, eliminates leading "shells"
   If $m $m=$m+' -' EndIf
  EndIf

  $D=$D/20	; reduce the divisor by power of 20 

 Loop

 ; If no digits were generated, return only a null shell "digit"
 $m=IIf($m='','-',$m)
 ;$m=IIf($m,$m,'-')	; LATE! -3 strokes

EndIf	; LATE! -5 strokes if removed

EndFunction
;!
;!
; end Mayan Number Converter
_________________________
Actually I am a Rocket Scientist! \:D