Is Altmann's memory-for-goals model valid for higher-level multitasking?
Short answer: 
Altmann's model is hierarchical and allows for "higher-level" goals and "lower-level" goals, but a goal is ultimately a goal―higher level goals are just sets of lower level goals, at least as far as I understand it. Given its ability to predict performance on solid laboratory tasks like the Tower of Hanoi, there is no reason to think that goal-activation/memory-for-goals theory wouldn't apply to "higher-level" multitasking.

Altmann's theory does not appear to be implemented in ACT-R. The relationship between the two instead appears to be that Altmann borrowed ACT-R's forgetting function for his model.

Longer answer: 
Goal-activation theory relates to real-world work in the sense that real-world work often requires juggling multiple goals. Such higher-level multitasking often requires the worker to temporarily stop working towards a goal in order to pursue another. Goal-activation theory helps explain this by accounting for this inhibition of the ongoing task and shifting to the new task. In order to do so, the worker must encode, store and retrieve different goals from memory depending on contextual salience and retrieval history.

The paper Jeff quotes in his comment, by Trafton, Altmann, Brock &amp; Mintz (2002), proposed two mechanisms for how different goals reach the top of the goal pile and actually become activated: prospective goal encoding and retrospective rehearsal. Both rely on the effect of retrieval practice, and possibly (but not explicitly stated) retrieval-induced forgetting. 

Participants were either interrupted during a task with no warning or an 8-second warning prior to interruption. They found that participants in the warning condition were able to improve their responses to interruption. This suggests that the goal-activation model's ability to explain inhibitory performance on Tower of Hanoi can be generalized to real world tasks.

(As an aside: Methodologically speaking, the Tower of Hanoi in particular is a generally well validated task for measuring executive inhibition across task domains. It has an impressive clinical/practical empirical pedigree that is readily available by, for instance, forward searching Miyake et al. (2000). I don't see why performance on a Tower of Hanoi task would be considered "low-level" or meaningfully different from "real-world knowledge work," at least in terms of aspects that might relate to goal-activation.)

References


Miyake, A., Friedman, N. P., Emerson, M. J., Witzki, A. H., Howerter, A., &amp; Wager, T. D. (2000). The unity and diversity of executive functions and their contributions to complex “frontal lobe” tasks: A latent variable analysis. Cognitive psychology, 41(1), 49-100. Chicago.
Trafton, J. G., Altmann, E. M., Brock, D. P., &amp; Mintz, F. E. (2003). Preparing to resume an interrupted task: Effects of prospective goal encoding and retrospective rehearsal. International Journal of Human-Computer Studies, 58(5), 583-603.