Is there any workaround to make use of html5 localstorage on both http and https?

Store all data on one domain, e.g. https://my.domain.org/.

On https protocols, simply use localStorage.setItem('key', 'value') to save the data.
On http protocols, embed a https frame, and use postMessage to save the data:

Demo: http://jsfiddle.net/gK7ce/4/ (with the helper page being located at http://jsfiddle.net/gK7ce/3/).

// Script at https://my.domain.org/postMessage
window.addEventListener('message', function(event) {
    // Domain restriction (to not leak variables to any page..)
    if (event.origin == 'http://my.domain.org' ||
        event.origin == 'https://my.domain.org') {
        var data = JSON.parse(event.data);
        if ('setItem' in data) {
            localStorage.setItem(data.setItem, data.value);
        } else if ('getItem' in data) {
            var gotItem = localStorage.getItem(data.getItem);
            // See below
            event.source.postMessage(
                '#localStorage#' + data.identifier + 
                (gotItem === null ? 'null#' : '#' + gotItem),
                event.origin
            );
        } else if ('removeItem' in data) {
            localStorage.removeItem(data.removeItem);
        }
    }
}, false);

On the http(s) page, the frame can be embedded as follows (replace https://my.mydomain.com with the actual URL. Note that you can simply get a reference to the frame, and use the src attribute):

<iframe name="myPostMessage" src="https://my.domain.org/postMessage" style="display:none;"></iframe>
// Example: Set the data
function LSsetItem(key, value) {
    var obj = {
        setItem: key,
        value: value
    };
    frames['myPostMessage'].postMessage(JSON.stringify(obj), 'https://my.domain.com');
}
LSsetItem('key', 'value');

Note that the method is asynchronous, because of postMessage. An implementation of the getItem method has to be implemented differently:

var callbacks = {};
window.addEventListener('message', function(event) {
    if (event.source === frames['myPostMessage']) {
        var data = /^#localStorage#(\d+)(null)?#([\S\s]*)/.exec(event.data);
        if (data) {
            if (callbacks[data[1]]) {
                // null and "null" are distinguished by our pattern
                callbacks[data[1]](data[2] === 'null' ? null : data[3]);
            }
            delete callbacks[data[1]];
        }
    }
}, false);
function LSgetItem(key, callback) {
    var identifier = new Date().getTime();
    var obj = {
        identifier: identifier,
        getItem: key
    };
    callbacks[identifier] = callback;
    frames['myPostMessage'].postMessage(JSON.stringify(obj), 'https://my.domain.com');
}
// Usage:
LSgetItem('key', function(value) {
    console.log('Value: ' + value);
});

Note that each callback is stored in a hash. Each message also contains an identifier, so that the window which receives the message calls the correct corresponding callback.

For completeness, here's the LSremoveItem method:

function LSremoveItem(key) {
    var obj = {
        removeItem: key
    };
    frames['myPostMessage'].postMessage(JSON.stringify(obj), 'https://my.domain.com');
}

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