Bob thanks a lot, you made me think again and I have solved the problem now. It makes me so happy, and that before the weekend :) Below a part of my solution, but I feel that the use of component and HTML/javascript in there feels a bit like a hack. I wonder how long Salesforce still allows this functionality. 

 For everybody who's interested. Here is what I did to transfer the standard country field into a picklist rather than a text field on standard account page. Next step for me is adding input fields so I can build postcode lookup :)

1) Enable the following setting: Go to Setup -> Customize -> User Interface -> Show Custom Sidebar Components on All Pages

2) Create a new home page component and add it to the narrow home page layout.

<script src="/js/dojo/0.4.1/dojo.js"></script>
<script src="/soap/ajax/19.0/connection.js" type="text/javascript"></script>
<script src="/resource/1280482442000/updatezip2" type="text/javascript"></script>
<script type="text/javascript">
	dojo.require("dojo.collections.Store");
	dojo.require("dojo.charting.Chart");
	dojo.require('dojo.json');
	dojo.addOnLoad(swapCountryText);
</script>

3) Upload the static resource with the javasript which can modify the page

var arCountries;
function swapCountryText(){
    // Account Billing Address
    if(document.getElementById('acc17country') != null) {
    	loadCountries();
        var select = document.getElementById('acc17country');
        var curValue = select.value; 
        var parentx = select.parentNode;
        parentx.removeChild(select);
        select = document.createElement('select');
        select.size = 1;
        select.id = 'acc17country';
        select.name = 'acc17country';
        parentx.appendChild(select);
    }

    // Account Shipping Address
    if(document.getElementById('acc18country') != null) {
    	loadCountries();
        var select2 = document.getElementById('acc18country');
        var curValue2 = select2.value; 
        var parenty = select2.parentNode;
        parenty.removeChild(select2);
        select2 = document.createElement('select');
        select2.size = 1;
        select2.id = 'acc18country';
        select2.name = 'acc18country';
        parenty.appendChild(select2);
    }
    if(select != null) {
        if(arCountries.length>0) {
            for(x=0;x<arCountries.length;x++) {
                if(arCountries[x] == curValue) {
                    select.options[x] = new Option(arCountries[x], arCountries[x], false, true);
                } else {
                    select.options[x] = new Option(arCountries[x], arCountries[x], false, false);
                }
            }
        } else {
                select.options[x] = new Option('No countries found', 'No countries found', false, true);
        }
    }
    if(select2 != null) {
        if(arCountries.length>0) {
            for(x=0;x < arCountries.length;x++) {
                if(arCountries[x] == curValue2) {
                    select2.options[x] = new Option(arCountries[x], arCountries[x], false, true);
                } else {
                    select2.options[x] = new Option(arCountries[x], arCountries[x], false, false);
                }
            }
        } else {
                select2.options[x] = new Option('No countries found', 'No countries found', false, true);
        }
    }
}

function loadCountries() {
	if(arCountries == null) {
		arCountries = getCountries();
	}
}

function getCountries() {
	sforce.sessionId = getCookie('sid');
	sforce.connection.sessionId=sforce.sessionId;
    var out = [];
    try {
        var queryCountries = sforce.connection.query("Select Id, Name FROM Country__c ORDER BY Name");  
        var countries = queryCountries.getArray('records');
        for(x=0;x<countries.length;x++) {
            out[out.length] = countries[x].Name;
    
        }   
                
    } catch(error) {
        alert(error);       
    }   
    return out;
}

 ps: thanks to Harm Korten which supplies most of this code http://salesforce.harmkorten.nl/2010/salesforce-country-fields-as-picklists/