It doesn't look like you'll be able to update this data without the field appearing on the layout, but with the Metadata API, you should be able to automate this.
I don't have a Professional Edition (PE) Org I can play around with, but based on previous knowledge, and what I can find in the docs, by default, PE Orgs don't have profiles, and as such, don't have Field Level Security (FLS). Profiles appear to be a paid Add-On for PE.
Salesforce Edition Comparison Chart
Furthermore, field visibility appears to be tied to the Page Layout. Any field that is not on the layout, is not visible.
Security Access in GE/PE
Both GE and PE do not support custom profiles. As a result, when a
customer installs your app, they will not be able to define which
profiles have access to what. Therefore, you should be sure to design
your app to work for the Standard User profile at a minimum.
Furthermore, these standard profiles are not editable, and field-level
security is handled on the page layout. In other words, any fields you
want to access to must be on the page layout or they will not be
visible by the API or Visualforce.
For instance, if you've create a custom Visualforce page that displays
a field, this field must be on the default page layout for it to
appear in the Visualforce page. The same is true with API integration.
You cannot read/write to a field that is not on the page layout in GE
Since you have API access and were able to create the field using the API, you should be able to retrieve, update, and deploy the Page Layout for that specific object. If you create the field & include that field in the layout, you should be to view/edit this fields data via the API.
Take a look at the Layout Section of the Metadata API Developer's Guide.