Like Dean said, there's no issue at all with non-reactive arguments in the module server function (reactive arguments in modules are just a very common and useful pattern).
For example, if you look at http://shiny.rstudio.com/gallery/module-example.html and copy-paste the module and the app.R files, you can see that non-reactives work fine by changing the cols argument in scatterPlot from left() to left (and from right() to right):
output$plot1 <- renderPlot({
scatterPlot(dataWithSelection(), left)
})
output$plot2 <- renderPlot({
scatterPlot(dataWithSelection(), right)
})
AND by changing the call to callModule in the app.R's server function from left = reactive(c("cty", "hwy")) to simply left = c("cty", "hwy") (and similarly for right):
df <- callModule(linkedScatter, "scatters", reactive(mpg),
left = c("cty", "hwy"),
right = c("drv", "hwy")
)
So, this wasn't the problem you were experiencing in your app...