Suppose the account currency and the instrument being traded are unknown at compile time:
What is the simplest way to open a position with amount = K*equity ?
(where K is a given constant)

Since order amounts are specified in the instrument's primary currency, while account equity is provided in account currency, I am thinking of using this messy conversion:

    private double getEquityInPrimaryCurrency(Instrument i) {
        Currency pri = i.getPrimaryCurrency();
        Currency acc = context.getAccount().getCurrency();
        
        double r;
        
        if(pri.equals(acc)){
            r = 1;
        } else {
            String priCode = pri.getCurrencyCode();
            String accCode = acc.getCurrencyCode();

            Instrument ratio = Instrument.fromString(priCode + '/' + accCode);
            try{
                if(ratio != null){
                    r = 1/context.getHistory().getLastTick(ratio).getBid();
                } else {
                    ratio = Instrument.fromString(accCode + '/' + priCode);
                    r = context.getHistory().getLastTick(ratio).getBid();
                }
            } catch (JFException ex){
                console.getErr().println("Cannot determine ratio between " + priCode + " and " + accCode + ". Try subscribing to the relevant instrument.");
                r = 0;
            }
        }
        return context.getAccount().getEquity()*r;
    }

And then to submit:

context.getEngine().submitOrder(label, instrument, orderCommand, K*getEquityInPrimaryCurrency(instrument))

But this is messy and sometimes would require subscription to an extra instrument. Is there in the API a better solution that I am missing?

No, at the current implementation there is not.