Determining the resulting pixel value before creating a compressed JPEG image
To give you an upfront answer your question you want to determine the $pixel-values$ from $DCT-Coefficients$ probably there is nothing on earth better than doing IDCT itself! 

However, leaving aside some vagueness i can re-phrase your question: 

If i have an algorithm x - that is defined to work on Pixels, here is what i do:
DCT-Coeffs ->  IDCT  -> pixels  Algorithm_X  -> pixels  DCT  -> DCT-Coeffs
(ignoring other stuff that is peripheral). 


  The question is: is there a short cut to this brute force where i am
  obviously undoing quite some stuff done in first stage?


What you are referring to is what is generally known as Compressed Domain Processing. 

DCT-Coeffs ->  Compressed_Domain_Algorithm_X  -> DCT-Coeffs

Basically, all compressed domain algorithm exploits the linearity of DCT. For example 

$$
Y = \alpha * X_1 + \beta * X_2  
$$
then,
$$
DCT(Y) = \alpha * DCT(X_1) + \beta * DCT(X_2) 
$$
where $\alpha$ $\beta$ are scalars. 

There are many things possible, and while many NOT. It is too vast to answer the set of all compressed domain algorithms which are possible, but i would recommend key readings that will set a great stage for you. 


Compressed Domain Processing of JPEG-encoded images by Brian C. Smith , Lawrence A. Rowe Real-Time Imaging (1996)
A Survey of Compressed Domain Processing Techniques by Brian C. Smith
Proc. NSF Workshop on Reconnecting Science
Algorithms for Manipulating Compressed Images, Brian C. Smith, Lawrence A. Rowe, 
IEEE Computer Graphics and Applications, Sept. 1993, vol.13, (no.5):34-42.
Manipulation and compositing of MC-DCT compressed video by Shih-Fu Chang, David G. Messerschmitt IEEE Journal on Selected Areas in Communications, 1995/1, Volume
13 Issue 1
Compressed-Domain Video Processing by Susie Wee, Bo Shen, John Apostolopoulo
HP Labs Tech Report 2002. 


Specifics: 
If you are using libjpeg you can actually get quantized DCT co-efficients directly. However, most above literature refers un-quantized DCT co-efficients.