I'm using the jquery UI datepicker control but do not want the 'keyboard' to display on mobile/tablet when a user clicks on the input field.
A way to prevent this is to add 'readyonly=readonly' to the input.
However, I'm concerned that this will negatively affect accessibility.
Will users, using a screen reader, still be able to edit the field / select a date.
The short answer is yes, it will be very difficult for visually impaired users to select a date.
In terms of screen readers, the experience will not too different if they are using NVDA and WindowEyes. Because these screen readers don't announce whether a text input is read-only or not. However, JAWS does announce if a field is read only. So it's a mixed bag and may cause confusion for some users.
But, more importantly, you should be aware that many blind users rely on keyboard to enter dates; picking a date from a table is a tedious task for them. So, disabling the keyboard may not be an ideal solution.
In my opinion it is best use a date picker icon instead of relying on the focus event. This way, sighted users can click on the icon to display the picker without ever displaying the keyboard.